AMERICAN PODIATRY ACCOCIATION

Axis of Motion of the Subtalar Joint

An Anatomical Study*
Editor: ABE RUBIN, D.S.C.

MERTON L. ROOT, D.S.C.
JOHN H. WEED, D.P.M.
THOMAS E. SGARLATO, D.P.M.
DANIEL R. BLUTH, D.P.M.

*Report of research conducted at Highland Alameda County Hospital, Oakland, Calif. Orthopedic Department, Podiatry College, San Francisco, Calif.

Published: April 1966

This study was conducted to refine the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot.

The authors' investigation reestablished the axis of motion of the subtalar joint in essentially the same position as described by previous investigators and supported the theories that the axis of motion is essentially perpendicular to the plane of motion and the subtalar joint is hinge-like in nature.

-

This investigation has been performed for the purpose of refining the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot. A modification of J. T. Manter's method of establishing the subtalar joint axis of motion was developed to expedite examination of this joint.

Terminology

Certain terms used in this paper are specifically defined as follows: axis of motiona plane around which a joint moves; plane of motion - a plane in which a joint moves; supination - a triplane motion of inversion, adduction and plantar flexion occurring simultaneously in a joint; pronation - a triplane
motion of eversion, abduction and dorsiflexion occurring simultaneously in a joint.

History

The average resultant axis of subtalar joint motion has been previously demonstrated.^1-4 Several earlier workers described the axis of motion but did not assign degrees of angulation from the various cardinal body planes. They gave a generalized description of its location; however, their description corresponds quite well with results of later investigation.

J. T. Manter¹ appears to be the first investigator to accurately establish and record the subtalar joint axis of motion in relationship to the cardinal planes of the body. The procedure developed in this study is a modification of his method. He used 16 specimens of cadaver feet which were dissected, leaving only the ligaments intact. His first step was to establish the plane of motion of the subtalar joint by inserting pins into the talus. The foot was then arranged so that the subtalar joint plane of motion was parallel to two parallel planes, dorsal and plantar to the foot, represented by two plates of glass on the opposite sides of the foot being studied. Another pin was then inserted into the talus parallel to the plane of motion with marking pins attached to it. These marking pins touched both plates of glass. The marking pins drew several arcs on each plate of glass when the subtalar joint was moved through its range of motion. The common center point of the arcs on each plane represented the point of transection of the axis of motion through that plane. The axis was then transposed obliquely across the foot from posterior, lateral and plantar to anterior, medial and dorsal. The resultant axis in the average foot in his series deviated from the sagittal plane by 16° and from the transverse plane by 42°.

J. H. Hicks² attempted to simulate physiologic conditions of the foot by leaving the foot undissected during his examination of the subtalar joint. He also attached previously calibrated tension springs to the long tendons duplicating muscle pull and applied a compression force to the tibia before establishing the axis of motion of the subtalar joint. He determined the axis by a trial and error method of inserting a pin into the talus along the axis of motion. The position of the pin was altered until only axial rotation of the pin occurred with motion of the subtalar joint. Of course, he had the advantage of knowing the results of Manter's work, so this method was much more efficient than it would have been if the joint had not been previously studied.

The function and range of motion of the subtalar joint was studied in gait by J. R. Close and V. T. Inman³ using three live subjects. They inserted a pin into the talus and calcaneus, then studied the motion which occurred between them. They developed a very accurate method of recording and reducing data. They were able to establish the relative position and motion of the calcaneus and talus in the various phases of gait. This functional relationship graphically demonstrates the position and function of the subtalar joint during each phase of gait. They demonstrated variations in subtalar joint function in flat, pronated and normal feet. Although the difference in these three types of feet were not specifically described, the criteria for classification of the feet of their subjects appears to have been the height and mobility of the medial longitudinal arch with weight bearing.

D. G. Wright, S. M. Desai and W. H. Henderson⁴ also studied the function of the subtalar joint in various conditions of gait. This group developed a device which was applied externally to the foot and leg to measure the direction and range of motion of the subtalar and ankle joints.

All investigators of the subtalar joint agree on the general direction and average deviation of the axis of motion computed by Manter. The axis described is the resultant axis around which subtalar joint motion occurs. This resultant axis is a combination of motion around an infinite number of axes present with each minute motion as stated by A. Steindler.⁶ Individual variations of the direction of the subtalar joint axis are noted in the findings of Manter and Hicks. These variations must be considered when examining each individual foot.

Figure 1

Figure 2

Methods and Materials

The feet of freshly amputated limbs which had no obvious necrotic or traumatic involvement of the tarsal or midtarsal areas were carefully dissected, leaving only the ligaments intact. Each foot was dissected and completely studied during one session to avoid the many possible complications of alteration in the physical properties of bone, cartilage and ligamentous tissue after exposure to the atmosphere. After a specimen was carefully prepared, it was mounted on a universal clamp by holding the posterior aspect of the calcaneus. Three marking pins, each of which was adjustable in length, were inserted into the body of the freely moveable talus at random angles (Fig. 1). With pronation and supination of the subtalar joint, the tip of each pin described an arc perpendicular to the axis of motion. If the motion of the subtalar joint is hinge-like in quality, as described by Hicks, then the arc described by each pin is parallel to the plane of motion of the subtalar joint and each arc is parallel to the other two arcs. Adjusting the length of each pin by trial and error, a point was found at which each pin not only described an arc parallel to the arcs created by the other two pins, but at that one point the arcs also remain on the same plane throughout the range of motion of the subtalar joint.

Experimentally, this one common plane to the motion of the tips of all three pins can be found by mounting a flat board on a second universal joint so that the board can be tilted in any direction (Fig. 1). The board was placed close to the tips of all three pins and the talus was moved at the subtalar joint. The three arcs created by the tips of the pins tended to move at an angle to the board. The board was then set so it was parallel to the arc of one pin and that pin became a constant. The other two pins were either lengthened or shortened until the arcs of their motion also paralleled the board. When the relative position of the board and pins was found so that the tips of all three pins remained in constant contact with the board throughout the excursion of the subtalar joint, the board then represented a plane that was parallel to the plane of motion of the subtalar joint and a perpendicular to the board represented the direction of the axis of motion (Fig. 2).

After the plane of motion was established with the board, the foot was placed in its anatomic position. We define this position as follows: (1) The sagittal plane of the calcaneus, which is determined by a vertical bisection of its posterior surface, is parallel to the sagittal plane of the foot. The sagittal plane of the foot is parallel to the sagittal plane of the body and therefore is perpendicular to the floor (transverse plane) when the normal foot is in the anatomic position. (2) A line from the plantar surface of the medial process of the calcaneal tuberosity to the plantar surface of the fifth metatarsal head is parallel to the transverse plane of the body (Fig. 2). This line represents the transverse plane of the foot. (3) The long axis of the foot, as used in this study, is represented by a line from the middle of the posterior superior edge of the calcaneus to the middle of the head of the second metatarsal (Fig. 1). This line is placed parallel to the sagittal plane of the body, since it represents the sagittal plane of the foot. We later realized that the choice of these anatomic landmarks, to establish the long axis of the foot, introduced unnecessary variables. This line is a poor reference because it crosses the tarsus, the lesser tarsus and the metatarsus without taking into consideration the transverse plane angular variation which occurs between these structural segments of the foot. The deviation of the long axis of the foot produced by changes in adductus of the metatarsus and abductus of the lesser tarsus was pointed out by T. E. Sgarlato.⁶ Since the relationship of the metatarsus adductus and the lesser tarsus abductus was not exactly the same in each foot examined, the sagittal plane of the rearfoot is deviated on the transverse plane, as one foot compared to another, by these variables. While the number of degrees of deviation is not usually great, they should be taken into consideration in future studies of the subtalar joint. Since several specimens had been studied using these landmarks, we continued using them to preserve continuity in this series.

When the universal clamp was secured with the specimen in the anatomic position, a three-sided box, the sides of which were perpendicular to the base, was placed around the foot. The base of the box was parallel to the transverse plane of the foot and the body. The sides of the box were parallel to the sagittal and frontal planes of the foot and the body (Figs. 1 and 2). The flat board which was attached to a ball and socket joint and a universal joint on a stand was placed inside the box with the foot. The plane of motion of the subtalar joint was reestablished, with the foot in the anatomic position, by positioning the board to meet the tips of the pins, which were adjusted previously to their proper relative lengths.

Figure 3

Figure 4

Table 1

Figure 5

Table 2

The arcs of motion produced by all three pins with subtalar joint motion remained parallel to the fl.at board. The foot was removed from the field without moving the board which represented the plane of motion of the subtalar joint (Fig. 3). The box, placed in relationship to the cardinal planes of the foot, represented the foot. The angulation of the board from the base of the box ( transverse plane of the foot) and lateral sides of the box (sagittal plane of the foot) was measured with a long goniometer, thus establishing the angulation of the plane of motion of the subtalar joint as it angles from the cardinal planes of the foot. The axis of motion being perpendicular to the plane of motion, was computed from the previously established plane of motion.

Manter disputes the accuracy of the statement "the axis of motion of the subtalar joint is perpendicular to the plane of motion" since he concluded the motion of the subtalar joint was screw-like in nature. He computed the helix angle of the subtalar joint to be 12°. The helix angle of a screw is determined by the pitch of its threads. Therefore, the plane of motion would angulate 12° from the axis of motion. However, the results of this investigation fail to confirm that the motion of the subtalar joint is screw-like in nature. Rather, it supports the hypothesis that the subtalar joint has hinge-like motion which occurs simultaneously on all three body planes. If any screw-like motion is present, the helix angle could not be greater than 1 to 2°. This deduction is possible since our data corresponds very closely with Manter's findings, even though we assumed the axis of motion to be perpendicular to the plane of motion in computing our results. This is substantiated by our findings.

Results

This investigation has reestablished the axis of motion -of the subtalar joint in essentially the same position as described by all previous investigators. The subtalar joint axis of the average foot, in this study, deviated from the sagittal plane from posterior lateral to anterior medial by 17° (Fig. 4), with a range of 8° to 29° (Table I). The axis deviated from the transverse plane from posterior plantar to anterior dorsal, in the average foot by 41 ° (Fig. 5) with a range of 22° to 55° (Table I). The standard deviation and the coefficient of variation were also computed (Table I). The raw data has been recorded in Table II.

The axis of motion of the subtalar joint as recorded in this study is similar to that recorded by Manter. This similarity tends to dispute the hypothesis that the motion of the subtalar joint is screw-like in nature, with a helix angle of more than 1 or 2°. It thus appears that the axis of motion is essentially perpendicular to the plane of motion, and the subtalar joint motion is hinge-like in nature. The consistency of results reestablishes the location of the axis of motion of the subtalar joint and indicates the accuracy of the method used for examination of this joint.

Discussion

An observation can be made as a result of this study as well as Manter's and Hick's work. The range of variance in the direction of the axis of subtalar joint motion should have considerable clinical significance. The greatest elevation of the anterior aspect of the axis from the transverse plane was 32° greater than the minimal angulation observed from the transverse plane in this series. The smallest angulation was 22° (Table II, Specimen 13) and the greatest angulation was 54° (Table II, Specimen 6). The individual with a greater angulation of the axis from the transverse plane would demonstrate greater than average adductionabduction motion of the foot at the subtalar joint when the motion of supination-pronation occurred at this joint in association with transverse plane rotation of the leg in gait.⁷

The person with a very low angulation of the axis from the transverse plane, conversely, presents greater than average inversion- eversion motion of the foot at the subtalar joint during supination-pronation associated with transverse plane leg rotation in gait.

Also, the range of elevation of the axis from the sagittal plane varied by 21 ° in the 22 feet examined. The smallest angulation was 8° (Table II, Specimen 1). As the axis deviates from the sagittal plane, greater (with greater angulation) or lesser (with smaller angulation) dorsiflexion-plantarflexion occurs at the subtalar joint with supination- pronation motion.

The extent of motion on each plane, namely, adduction-abduction (transverse plane), would vary from one person to another as the axis varied in relation to all three body planes.

Since during the stance phase of gait, supination-pronation of the subtalar joint occurs in conjunction with transverse plane rotation of the leg at the hip, and since inversion-eversion motion occurs in the foot while the talus and leg move simultaneously into adduction-abduction and plantarflexion-dorsiflexion, it can be deduced that motion of the foot in gait is much greater in some people than in others even though the transverse plane rotation of the leg may be equal in these people.

Summary

The location of the axis of motion of the subtalar joint has been reconfirmed as earlier recorded by others. A new method of establishing the axis of motion of the subtalar joint in cadaver feet has been developed. The hypothesis that the subtalar joint motion is screw-like in nature with a helix angle of more than 1 to 2° is disputed. Hick's hypothesis that motion of the subtalar joint is hinge-like in nature, occurring simultaneously in all three body planes is supported. The importance of individual variation in the direction of axis of motion of the subtalar joint in its effect on motion in the foot during gait was discussed.

Acknowledgment. The amputated limbs used in this study were obtained through the cooperation of the Pathology Department and with the sponsorship of the Podiatry Department, Highland Alameda County Hospital, Oakland, Calif.

1848 Saratoga Ave.
San Jose, California

References

1. MANTER, J. T.: Movements of the subtalar and transverse tarsal joints. Anat. Rec., 80: 397, 1941.
2. HICKS, J. H.: The mechanics of the foot. J. Anat., 87: 345, 1953.
3. CLOSE, J. R. AND INMAN, V. T.: The action of the subtalar joint. Prosthetic Devices Research Project, Institute of Engineering Research, University of California at Berkeley, Series II, Issue 24, May 1953.
4. WRIGHT, D. G., DESAI, s. M. AND HENDERSON, W. H.: Action of the subtalar and ankle joint complex during the stance phase of walking. J. Bone Joint Surg., 46A: 361, 1964.
5. STEINDLER, A.: Kinesiology. Charles C Thomas, Springfield, Ill., 1955.
6. SGARLATOT, . E.: The angle of gait. J.A.P.A., 55: 645, 1965.
7. LEVENS, A. s., INMAN, v. T. AND BLOSSER, J. A.: Transverse rotation of the segments of the lower extremity in locomotion. J. Bone Joint Surg., 30A: 859, 1948.

AMERICAN PODIATRY ACCOCIATION

An Anatomical Study*
Editor: ABE RUBIN, D.S.C.

MERTON L. ROOT, D.S.C.
JOHN H. WEED, D.P.M.
THOMAS E. SGARLATO, D.P.M.
DANIEL R. BLUTH, D.P.M.

*Report of research conducted at Highland Alameda County Hospital, Oakland, Calif. Orthopedic Department, Podiatry College, San Francisco, Calif.

Published: April 1966

Axis of Motion of the Subtalar Joint

This study was conducted to refine the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot.

The authors' investigation reestablished the axis of motion of the subtalar joint in essentially the same position as described by previous investigators and supported the theories that the axis of motion is essentially perpendicular to the plane of motion and the subtalar joint is hinge-like in nature.

-

This investigation has been performed for the purpose of refining the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot. A modification of J. T. Manter's method of establishing the subtalar joint axis of motion was developed to expedite examination of this joint.

Terminology

Certain terms used in this paper are specifically defined as follows: axis of motiona plane around which a joint moves; plane of motion - a plane in which a joint moves; supination - a triplane motion of inversion, adduction and plantar flexion occurring simultaneously in a joint; pronation - a triplane
motion of eversion, abduction and dorsiflexion occurring simultaneously in a joint.

History

The average resultant axis of subtalar joint motion has been previously demonstrated.^1-4 Several earlier workers described the axis of motion but did not assign degrees of angulation from the various cardinal body planes. They gave a generalized description of its location; however, their description corresponds quite well with results of later investigation.

J. T. Manter¹ appears to be the first investigator to accurately establish and record the subtalar joint axis of motion in relationship to the cardinal planes of the body. The procedure developed in this study is a modification of his method. He used 16 specimens of cadaver feet which were dissected, leaving only the ligaments intact. His first step was to establish the plane of motion of the subtalar joint by inserting pins into the talus. The foot was then arranged so that the subtalar joint plane of motion was parallel to two parallel planes, dorsal and plantar to the foot, represented by two plates of glass on the opposite sides of the foot being studied. Another pin was then inserted into the talus parallel to the plane of motion with marking pins attached to it. These marking pins touched both plates of glass. The marking pins drew several arcs on each plate of glass when the subtalar joint was moved through its range of motion. The common center point of the arcs on each plane represented the point of transection of the axis of motion through that plane. The axis was then transposed obliquely across the foot from posterior, lateral and plantar to anterior, medial and dorsal. The resultant axis in the average foot in his series deviated from the sagittal plane by 16° and from the transverse plane by 42°.

J. H. Hicks² attempted to simulate physiologic conditions of the foot by leaving the foot undissected during his examination of the subtalar joint. He also attached previously calibrated tension springs to the long tendons duplicating muscle pull and applied a compression force to the tibia before establishing the axis of motion of the subtalar joint. He determined the axis by a trial and error method of inserting a pin into the talus along the axis of motion. The position of the pin was altered until only axial rotation of the pin occurred with motion of the subtalar joint. Of course, he had the advantage of knowing the results of Manter's work, so this method was much more efficient than it would have been if the joint had not been previously studied.

The function and range of motion of the subtalar joint was studied in gait by J. R. Close and V. T. Inman³ using three live subjects. They inserted a pin into the talus and calcaneus, then studied the motion which occurred between them. They developed a very accurate method of recording and reducing data. They were able to establish the relative position and motion of the calcaneus and talus in the various phases of gait. This functional relationship graphically demonstrates the position and function of the subtalar joint during each phase of gait. They demonstrated variations in subtalar joint function in flat, pronated and normal feet. Although the difference in these three types of feet were not specifically described, the criteria for classification of the feet of their subjects appears to have been the height and mobility of the medial longitudinal arch with weight bearing.

D. G. Wright, S. M. Desai and W. H. Henderson⁴ also studied the function of the subtalar joint in various conditions of gait. This group developed a device which was applied externally to the foot and leg to measure the direction and range of motion of the subtalar and ankle joints.

All investigators of the subtalar joint agree on the general direction and average deviation of the axis of motion computed by Manter. The axis described is the resultant axis around which subtalar joint motion occurs. This resultant axis is a combination of motion around an infinite number of axes present with each minute motion as stated by A. Steindler.⁶ Individual variations of the direction of the subtalar joint axis are noted in the findings of Manter and Hicks. These variations must be considered when examining each individual foot.

Figure 1

Figure 2

Methods and Materials

The feet of freshly amputated limbs which had no obvious necrotic or traumatic involvement of the tarsal or midtarsal areas were carefully dissected, leaving only the ligaments intact. Each foot was dissected and completely studied during one session to avoid the many possible complications of alteration in the physical properties of bone, cartilage and ligamentous tissue after exposure to the atmosphere. After a specimen was carefully prepared, it was mounted on a universal clamp by holding the posterior aspect of the calcaneus. Three marking pins, each of which was adjustable in length, were inserted into the body of the freely moveable talus at random angles (Fig. 1). With pronation and supination of the subtalar joint, the tip of each pin described an arc perpendicular to the axis of motion. If the motion of the subtalar joint is hinge-like in quality, as described by Hicks, then the arc described by each pin is parallel to the plane of motion of the subtalar joint and each arc is parallel to the other two arcs. Adjusting the length of each pin by trial and error, a point was found at which each pin not only described an arc parallel to the arcs created by the other two pins, but at that one point the arcs also remain on the same plane throughout the range of motion of the subtalar joint.

Experimentally, this one common plane to the motion of the tips of all three pins can be found by mounting a flat board on a second universal joint so that the board can be tilted in any direction (Fig. 1). The board was placed close to the tips of all three pins and the talus was moved at the subtalar joint. The three arcs created by the tips of the pins tended to move at an angle to the board. The board was then set so it was parallel to the arc of one pin and that pin became a constant. The other two pins were either lengthened or shortened until the arcs of their motion also paralleled the board. When the relative position of the board and pins was found so that the tips of all three pins remained in constant contact with the board throughout the excursion of the subtalar joint, the board then represented a plane that was parallel to the plane of motion of the subtalar joint and a perpendicular to the board represented the direction of the axis of motion (Fig. 2).

After the plane of motion was established with the board, the foot was placed in its anatomic position. We define this position as follows: (1) The sagittal plane of the calcaneus, which is determined by a vertical bisection of its posterior surface, is parallel to the sagittal plane of the foot. The sagittal plane of the foot is parallel to the sagittal plane of the body and therefore is perpendicular to the floor (transverse plane) when the normal foot is in the anatomic position. (2) A line from the plantar surface of the medial process of the calcaneal tuberosity to the plantar surface of the fifth metatarsal head is parallel to the transverse plane of the body (Fig. 2). This line represents the transverse plane of the foot. (3) The long axis of the foot, as used in this study, is represented by a line from the middle of the posterior superior edge of the calcaneus to the middle of the head of the second metatarsal (Fig. 1). This line is placed parallel to the sagittal plane of the body, since it represents the sagittal plane of the foot. We later realized that the choice of these anatomic landmarks, to establish the long axis of the foot, introduced unnecessary variables. This line is a poor reference because it crosses the tarsus, the lesser tarsus and the metatarsus without taking into consideration the transverse plane angular variation which occurs between these structural segments of the foot. The deviation of the long axis of the foot produced by changes in adductus of the metatarsus and abductus of the lesser tarsus was pointed out by T. E. Sgarlato.⁶ Since the relationship of the metatarsus adductus and the lesser tarsus abductus was not exactly the same in each foot examined, the sagittal plane of the rearfoot is deviated on the transverse plane, as one foot compared to another, by these variables. While the number of degrees of deviation is not usually great, they should be taken into consideration in future studies of the subtalar joint. Since several specimens had been studied using these landmarks, we continued using them to preserve continuity in this series.

When the universal clamp was secured with the specimen in the anatomic position, a three-sided box, the sides of which were perpendicular to the base, was placed around the foot. The base of the box was parallel to the transverse plane of the foot and the body. The sides of the box were parallel to the sagittal and frontal planes of the foot and the body (Figs. 1 and 2). The flat board which was attached to a ball and socket joint and a universal joint on a stand was placed inside the box with the foot. The plane of motion of the subtalar joint was reestablished, with the foot in the anatomic position, by positioning the board to meet the tips of the pins, which were adjusted previously to their proper relative lengths.

Figure 3

Figure 4

Table 1

Figure 5

Table 1

The arcs of motion produced by all three pins with subtalar joint motion remained parallel to the fl.at board. The foot was removed from the field without moving the board which represented the plane of motion of the subtalar joint (Fig. 3). The box, placed in relationship to the cardinal planes of the foot, represented the foot. The angulation of the board from the base of the box ( transverse plane of the foot) and lateral sides of the box (sagittal plane of the foot) was measured with a long goniometer, thus establishing the angulation of the plane of motion of the subtalar joint as it angles from the cardinal planes of the foot. The axis of motion being perpendicular to the plane of motion, was computed from the previously established plane of motion.

Manter disputes the accuracy of the statement "the axis of motion of the subtalar joint is perpendicular to the plane of motion" since he concluded the motion of the subtalar joint was screw-like in nature. He computed the helix angle of the subtalar joint to be 12°. The helix angle of a screw is determined by the pitch of its threads. Therefore, the plane of motion would angulate 12° from the axis of motion. However, the results of this investigation fail to confirm that the motion of the subtalar joint is screw-like in nature. Rather, it supports the hypothesis that the subtalar joint has hinge-like motion which occurs simultaneously on all three body planes. If any screw-like motion is present, the helix angle could not be greater than 1 to 2°. This deduction is possible since our data corresponds very closely with Manter's findings, even though we assumed the axis of motion to be perpendicular to the plane of motion in computing our results. This is substantiated by our findings.

Results

This investigation has reestablished the axis of motion -of the subtalar joint in essentially the same position as described by all previous investigators. The subtalar joint axis of the average foot, in this study, deviated from the sagittal plane from posterior lateral to anterior medial by 17° (Fig. 4), with a range of 8° to 29° (Table I). The axis deviated from the transverse plane from posterior plantar to anterior dorsal, in the average foot by 41 ° (Fig. 5) with a range of 22° to 55° (Table I). The standard deviation and the coefficient of variation were also computed (Table I). The raw data has been recorded in Table II.

The axis of motion of the subtalar joint as recorded in this study is similar to that recorded by Manter. This similarity tends to dispute the hypothesis that the motion of the subtalar joint is screw-like in nature, with a helix angle of more than 1 or 2°. It thus appears that the axis of motion is essentially perpendicular to the plane of motion, and the subtalar joint motion is hinge-like in nature. The consistency of results reestablishes the location of the axis of motion of the subtalar joint and indicates the accuracy of the method used for examination of this joint.

Discussion

An observation can be made as a result of this study as well as Manter's and Hick's work. The range of variance in the direction of the axis of subtalar joint motion should have considerable clinical significance. The greatest elevation of the anterior aspect of the axis from the transverse plane was 32° greater than the minimal angulation observed from the transverse plane in this series. The smallest angulation was 22° (Table II, Specimen 13) and the greatest angulation was 54° (Table II, Specimen 6). The individual with a greater angulation of the axis from the transverse plane would demonstrate greater than average adductionabduction motion of the foot at the subtalar joint when the motion of supination-pronation occurred at this joint in association with transverse plane rotation of the leg in gait.⁷

The person with a very low angulation of the axis from the transverse plane, conversely, presents greater than average inversion- eversion motion of the foot at the subtalar joint during supination-pronation associated with transverse plane leg rotation in gait.

Also, the range of elevation of the axis from the sagittal plane varied by 21 ° in the 22 feet examined. The smallest angulation was 8° (Table II, Specimen 1). As the axis deviates from the sagittal plane, greater (with greater angulation) or lesser (with smaller angulation) dorsiflexion-plantarflexion occurs at the subtalar joint with supination- pronation motion.

The extent of motion on each plane, namely, adduction-abduction (transverse plane), would vary from one person to another as the axis varied in relation to all three body planes.

Since during the stance phase of gait, supination-pronation of the subtalar joint occurs in conjunction with transverse plane rotation of the leg at the hip, and since inversion-eversion motion occurs in the foot while the talus and leg move simultaneously into adduction-abduction and plantarflexion-dorsiflexion, it can be deduced that motion of the foot in gait is much greater in some people than in others even though the transverse plane rotation of the leg may be equal in these people.

Summary

The location of the axis of motion of the subtalar joint has been reconfirmed as earlier recorded by others. A new method of establishing the axis of motion of the subtalar joint in cadaver feet has been developed. The hypothesis that the subtalar joint motion is screw-like in nature with a helix angle of more than 1 to 2° is disputed. Hick's hypothesis that motion of the subtalar joint is hinge-like in nature, occurring simultaneously in all three body planes is supported. The importance of individual variation in the direction of axis of motion of the subtalar joint in its effect on motion in the foot during gait was discussed.

Acknowledgment. The amputated limbs used in this study were obtained through the cooperation of the Pathology Department and with the sponsorship of the Podiatry Department, Highland Alameda County Hospital, Oakland, Calif.

1848 Saratoga Ave.
San Jose, California

References

1. MANTER, J. T.: Movements of the subtalar and transverse tarsal joints. Anat. Rec., 80: 397, 1941.
2. HICKS, J. H.: The mechanics of the foot. J. Anat., 87: 345, 1953.
3. CLOSE, J. R. AND INMAN, V. T.: The action of the subtalar joint. Prosthetic Devices Research Project, Institute of Engineering Research, University of California at Berkeley, Series II, Issue 24, May 1953.
4. WRIGHT, D. G., DESAI, s. M. AND HENDERSON, W. H.: Action of the subtalar and ankle joint complex during the stance phase of walking. J. Bone Joint Surg., 46A: 361, 1964.
5. STEINDLER, A.: Kinesiology. Charles C Thomas, Springfield, Ill., 1955.
6. SGARLATOT, . E.: The angle of gait. J.A.P.A., 55: 645, 1965.
7. LEVENS, A. s., INMAN, v. T. AND BLOSSER, J. A.: Transverse rotation of the segments of the lower extremity in locomotion. J. Bone Joint Surg., 30A: 859, 1948.

AMERICAN PODIATRY ACCOCIATION

An Anatomical Study*
Editor: ABE RUBIN, D.S.C.

MERTON L. ROOT, D.S.C.
JOHN H. WEED, D.P.M.
THOMAS E. SGARLATO, D.P.M.
DANIEL R. BLUTH, D.P.M.

*Report of research conducted at Highland Alameda County Hospital, Oakland, Calif. Orthopedic Department, Podiatry College, San Francisco, Calif.

Published: April 1966

Axis of Motion of the Subtalar Joint

This study was conducted to refine the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot.

The authors' investigation reestablished the axis of motion of the subtalar joint in essentially the same position as described by previous investigators and supported the theories that the axis of motion is essentially perpendicular to the plane of motion and the subtalar joint is hinge-like in nature.

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This investigation has been performed for the purpose of refining the present knowledge of the subtalar joint by studying the angulation of its axis of motion from the transverse, sagittal and frontal planes of the foot. A modification of J. T. Manter's method of establishing the subtalar joint axis of motion was developed to expedite examination of this joint.

Terminology

Certain terms used in this paper are specifically defined as follows: axis of motiona plane around which a joint moves; plane of motion - a plane in which a joint moves; supination - a triplane motion of inversion, adduction and plantar flexion occurring simultaneously in a joint; pronation - a triplane
motion of eversion, abduction and dorsiflexion occurring simultaneously in a joint.

History

The average resultant axis of subtalar joint motion has been previously demonstrated.^1-4 Several earlier workers described the axis of motion but did not assign degrees of angulation from the various cardinal body planes. They gave a generalized description of its location; however, their description corresponds quite well with results of later investigation.

J. T. Manter¹ appears to be the first investigator to accurately establish and record the subtalar joint axis of motion in relationship to the cardinal planes of the body. The procedure developed in this study is a modification of his method. He used 16 specimens of cadaver feet which were dissected, leaving only the ligaments intact. His first step was to establish the plane of motion of the subtalar joint by inserting pins into the talus. The foot was then arranged so that the subtalar joint plane of motion was parallel to two parallel planes, dorsal and plantar to the foot, represented by two plates of glass on the opposite sides of the foot being studied. Another pin was then inserted into the talus parallel to the plane of motion with marking pins attached to it. These marking pins touched both plates of glass. The marking pins drew several arcs on each plate of glass when the subtalar joint was moved through its range of motion. The common center point of the arcs on each plane represented the point of transection of the axis of motion through that plane. The axis was then transposed obliquely across the foot from posterior, lateral and plantar to anterior, medial and dorsal. The resultant axis in the average foot in his series deviated from the sagittal plane by 16° and from the transverse plane by 42°.

J. H. Hicks² attempted to simulate physiologic conditions of the foot by leaving the foot undissected during his examination of the subtalar joint. He also attached previously calibrated tension springs to the long tendons duplicating muscle pull and applied a compression force to the tibia before establishing the axis of motion of the subtalar joint. He determined the axis by a trial and error method of inserting a pin into the talus along the axis of motion. The position of the pin was altered until only axial rotation of the pin occurred with motion of the subtalar joint. Of course, he had the advantage of knowing the results of Manter's work, so this method was much more efficient than it would have been if the joint had not been previously studied.

The function and range of motion of the subtalar joint was studied in gait by J. R. Close and V. T. Inman³ using three live subjects. They inserted a pin into the talus and calcaneus, then studied the motion which occurred between them. They developed a very accurate method of recording and reducing data. They were able to establish the relative position and motion of the calcaneus and talus in the various phases of gait. This functional relationship graphically demonstrates the position and function of the subtalar joint during each phase of gait. They demonstrated variations in subtalar joint function in flat, pronated and normal feet. Although the difference in these three types of feet were not specifically described, the criteria for classification of the feet of their subjects appears to have been the height and mobility of the medial longitudinal arch with weight bearing.

D. G. Wright, S. M. Desai and W. H. Henderson⁴ also studied the function of the subtalar joint in various conditions of gait. This group developed a device which was applied externally to the foot and leg to measure the direction and range of motion of the subtalar and ankle joints.

All investigators of the subtalar joint agree on the general direction and average deviation of the axis of motion computed by Manter. The axis described is the resultant axis around which subtalar joint motion occurs. This resultant axis is a combination of motion around an infinite number of axes present with each minute motion as stated by A. Steindler.⁶ Individual variations of the direction of the subtalar joint axis are noted in the findings of Manter and Hicks. These variations must be considered when examining each individual foot.

Methods and Materials

The feet of freshly amputated limbs which had no obvious necrotic or traumatic involvement of the tarsal or midtarsal areas were carefully dissected, leaving only the ligaments intact. Each foot was dissected and completely studied during one session to avoid the many possible complications of alteration in the physical properties of bone, cartilage and ligamentous tissue after exposure to the atmosphere. After a specimen was carefully prepared, it was mounted on a universal clamp by holding the posterior aspect of the calcaneus. Three marking pins, each of which was adjustable in length, were inserted into the body of the freely moveable talus at random angles (Fig. 1).

Figure 1

With pronation and supination of the subtalar joint, the tip of each pin described an arc perpendicular to the axis of motion. If the motion of the subtalar joint is hinge-like in quality, as described by Hicks, then the arc described by each pin is parallel to the plane of motion of the subtalar joint and each arc is parallel to the other two arcs. Adjusting the length of each pin by trial and error, a point was found at which each pin not only described an arc parallel to the arcs created by the other two pins, but at that one point the arcs also remain on the same plane throughout the range of motion of the subtalar joint.

Experimentally, this one common plane to the motion of the tips of all three pins can be found by mounting a flat board on a second universal joint so that the board can be tilted in any direction (Fig. 1). The board was placed close to the tips of all three pins and the talus was moved at the subtalar joint. The three arcs created by the tips of the pins tended to move at an angle to the board. The board was then set so it was parallel to the arc of one pin and that pin became a constant. The other two pins were either lengthened or shortened until the arcs of their motion also paralleled the board. When the relative position of the board and pins was found so that the tips of all three pins remained in constant contact with the board throughout the excursion of the subtalar joint, the board then represented a plane that was parallel to the plane of motion of the subtalar joint and a perpendicular to the board represented the direction of the axis of motion (Fig. 2).

Figure 2

After the plane of motion was established with the board, the foot was placed in its anatomic position. We define this position as follows: (1) The sagittal plane of the calcaneus, which is determined by a vertical bisection of its posterior surface, is parallel to the sagittal plane of the foot. The sagittal plane of the foot is parallel to the sagittal plane of the body and therefore is perpendicular to the floor (transverse plane) when the normal foot is in the anatomic position. (2) A line from the plantar surface of the medial process of the calcaneal tuberosity to the plantar surface of the fifth metatarsal head is parallel to the transverse plane of the body (Fig. 2). This line represents the transverse plane of the foot. (3) The long axis of the foot, as used in this study, is represented by a line from the middle of the posterior superior edge of the calcaneus to the middle of the head of the second metatarsal (Fig. 1). This line is placed parallel to the sagittal plane of the body, since it represents the sagittal plane of the foot. We later realized that the choice of these anatomic landmarks, to establish the long axis of the foot, introduced unnecessary variables. This line is a poor reference because it crosses the tarsus, the lesser tarsus and the metatarsus without taking into consideration the transverse plane angular variation which occurs between these structural segments of the foot. The deviation of the long axis of the foot produced by changes in adductus of the metatarsus and abductus of the lesser tarsus was pointed out by T. E. Sgarlato.⁶ Since the relationship of the metatarsus adductus and the lesser tarsus abductus was not exactly the same in each foot examined, the sagittal plane of the rearfoot is deviated on the transverse plane, as one foot compared to another, by these variables. While the number of degrees of deviation is not usually great, they should be taken into consideration in future studies of the subtalar joint. Since several specimens had been studied using these landmarks, we continued using them to preserve continuity in this series.

When the universal clamp was secured with the specimen in the anatomic position, a three-sided box, the sides of which were perpendicular to the base, was placed around the foot. The base of the box was parallel to the transverse plane of the foot and the body. The sides of the box were parallel to the sagittal and frontal planes of the foot and the body (Figs. 1 and 2). The flat board which was attached to a ball and socket joint and a universal joint on a stand was placed inside the box with the foot. The plane of motion of the subtalar joint was reestablished, with the foot in the anatomic position, by positioning the board to meet the tips of the pins, which were adjusted previously to their proper relative lengths.

Figure 3

The arcs of motion produced by all three pins with subtalar joint motion remained parallel to the fl.at board. The foot was removed from the field without moving the board which represented the plane of motion of the subtalar joint (Fig. 3). The box, placed in relationship to the cardinal planes of the foot, represented the foot. The angulation of the board from the base of the box ( transverse plane of the foot) and lateral sides of the box (sagittal plane of the foot) was measured with a long goniometer, thus establishing the angulation of the plane of motion of the subtalar joint as it angles from the cardinal planes of the foot. The axis of motion being perpendicular to the plane of motion, was computed from the previously established plane of motion.

Manter disputes the accuracy of the statement "the axis of motion of the subtalar joint is perpendicular to the plane of motion" since he concluded the motion of the subtalar joint was screw-like in nature. He computed the helix angle of the subtalar joint to be 12°. The helix angle of a screw is determined by the pitch of its threads. Therefore, the plane of motion would angulate 12° from the axis of motion. However, the results of this investigation fail to confirm that the motion of the subtalar joint is screw-like in nature. Rather, it supports the hypothesis that the subtalar joint has hinge-like motion which occurs simultaneously on all three body planes. If any screw-like motion is present, the helix angle could not be greater than 1 to 2°. This deduction is possible since our data corresponds very closely with Manter's findings, even though we assumed the axis of motion to be perpendicular to the plane of motion in computing our results. This is substantiated by our findings.

Results

This investigation has reestablished the axis of motion -of the subtalar joint in essentially the same position as described by all previous investigators. The subtalar joint axis of the average foot, in this study, deviated from the sagittal plane from posterior lateral to anterior medial by 17° (Fig. 4), with a range of 8° to 29° (Table I).

Figure 4

Table 1

The axis deviated from the transverse plane from posterior plantar to anterior dorsal, in the average foot by 41 ° (Fig. 5) with a range of 22° to 55° (Table I). The standard deviation and the coefficient of variation were also computed (Table I). The raw data has been recorded in Table II.

Figure 5

Table 2

The axis of motion of the subtalar joint as recorded in this study is similar to that recorded by Manter. This similarity tends to dispute the hypothesis that the motion of the subtalar joint is screw-like in nature, with a helix angle of more than 1 or 2°. It thus appears that the axis of motion is essentially perpendicular to the plane of motion, and the subtalar joint motion is hinge-like in nature. The consistency of results reestablishes the location of the axis of motion of the subtalar joint and indicates the accuracy of the method used for examination of this joint.

Discussion

An observation can be made as a result of this study as well as Manter's and Hick's work. The range of variance in the direction of the axis of subtalar joint motion should have considerable clinical significance. The greatest elevation of the anterior aspect of the axis from the transverse plane was 32° greater than the minimal angulation observed from the transverse plane in this series. The smallest angulation was 22° (Table II, Specimen 13) and the greatest angulation was 54° (Table II, Specimen 6). The individual with a greater angulation of the axis from the transverse plane would demonstrate greater than average adductionabduction motion of the foot at the subtalar joint when the motion of supination-pronation occurred at this joint in association with transverse plane rotation of the leg in gait.⁷

The person with a very low angulation of the axis from the transverse plane, conversely, presents greater than average inversion- eversion motion of the foot at the subtalar joint during supination-pronation associated with transverse plane leg rotation in gait.

Also, the range of elevation of the axis from the sagittal plane varied by 21 ° in the 22 feet examined. The smallest angulation was 8° (Table II, Specimen 1). As the axis deviates from the sagittal plane, greater (with greater angulation) or lesser (with smaller angulation) dorsiflexion-plantarflexion occurs at the subtalar joint with supination- pronation motion.

The extent of motion on each plane, namely, adduction-abduction (transverse plane), would vary from one person to another as the axis varied in relation to all three body planes.

Since during the stance phase of gait, supination-pronation of the subtalar joint occurs in conjunction with transverse plane rotation of the leg at the hip, and since inversion-eversion motion occurs in the foot while the talus and leg move simultaneously into adduction-abduction and plantarflexion-dorsiflexion, it can be deduced that motion of the foot in gait is much greater in some people than in others even though the transverse plane rotation of the leg may be equal in these people.

Summary

The location of the axis of motion of the subtalar joint has been reconfirmed as earlier recorded by others. A new method of establishing the axis of motion of the subtalar joint in cadaver feet has been developed. The hypothesis that the subtalar joint motion is screw-like in nature with a helix angle of more than 1 to 2° is disputed. Hick's hypothesis that motion of the subtalar joint is hinge-like in nature, occurring simultaneously in all three body planes is supported. The importance of individual variation in the direction of axis of motion of the subtalar joint in its effect on motion in the foot during gait was discussed.

Acknowledgment. The amputated limbs used in this study were obtained through the cooperation of the Pathology Department and with the sponsorship of the Podiatry Department, Highland Alameda County Hospital, Oakland, Calif.

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References

1. MANTER, J. T.: Movements of the subtalar and transverse tarsal joints. Anat. Rec., 80: 397, 1941.
2. HICKS, J. H.: The mechanics of the foot. J. Anat., 87: 345, 1953.
3. CLOSE, J. R. AND INMAN, V. T.: The action of the subtalar joint. Prosthetic Devices Research Project, Institute of Engineering Research, University of California at Berkeley, Series II, Issue 24, May 1953.
4. WRIGHT, D. G., DESAI, s. M. AND HENDERSON, W. H.: Action of the subtalar and ankle joint complex during the stance phase of walking. J. Bone Joint Surg., 46A: 361, 1964.
5. STEINDLER, A.: Kinesiology. Charles C Thomas, Springfield, Ill., 1955.
6. SGARLATOT, . E.: The angle of gait. J.A.P.A., 55: 645, 1965.
7. LEVENS, A. s., INMAN, v. T. AND BLOSSER, J. A.: Transverse rotation of the segments of the lower extremity in locomotion. J. Bone Joint Surg., 30A: 859, 1948.