BIOMECHANICAL ASSESSMENT OF A NEW TENODESIS FOR CORRECTION OF HALLUX VARUS
February 13th, 1995
Paul J. Juliano, MD; Mark S. Myerson, MD; and Bryan W. Cunningham, MS
Abstract
Each of six below-the-knee amputation specimens were transfixed to a wooden block and mounted to a jig on an MTS amputee testing device preloaded with 5 N applied to the proximal phalanx and displaced at a constant rate of 2 mm/min. Load displacement curves were generated for the intact joint and after sequential incisions of the lateral capsule, the adductor hallucis, and the lateral slip of the flexor hallucis brevis tendon, which caused varus dislocation of the hallux. An extensor hallucis brevis tenodesis was performed after the varus dislocation. Division of the lateral capsule, the adductor, and the flexor brevis reduced the force required to displace the hallux by 42.2%, an additional 25.2%, and a further 14.2%, respectively. Use of the extensor hallucis brevis tenodesis restored the load displacement curves to that of the normal joint. We concluded that the extensor hallucis brevis tendon may be useful as a tenodesis for reconstructing the deformity of acquired hallux varus.
Introduction
The functional balance of the hallux metatarsophalangeal (MP) joint is well understood in both normal and pathologic conditions.3,4,6,10 The four short muscles that insert into the base of the proximal phalanx of the hallux provide this functional equilibrium and, under normal circumstances, allow no more than 15o of lateral deviation of the hallux.4,15,20 The conjoined adductor and flexor hallucis brevis tendons presumably provide both static and dynamic stability to medially directed forces on the hallux. The normal restraints to hallux varus, however, have not been well documented, since the dynamic forces about the hallux MP joint have been largely inferred from the stages of hallux valgus deformity.6,10
Many operations are available to correct acquired hallux varus.9,16,18,23 Although arthrodesis or arthroplasty of the MP joint correct alignment, a tendon transfer can correct the deformity and simultaneously maintain the dynamic balance of the MP joint. The extensor hallucis longus (EHL) transfer described by Johnson is one option for correction.8,9 This technique, however, compromises the function of the EHL and, therefore, dorsiflexion of the hallux. In an effort to avoid these potential complications, the senior author (MM) developed a transfer of the extensor hallucis brevis (EHB) that functions as a tenodesis but does not disrupt the function of the EHL.
This study was therefore designed to determine the anatomic restraints to hallux varus and to assess the efficacy of the EHB as a tenodesis to correct this deformity in the cadaver model.
Materials and Methods
Six below-the-knee amputation specimens were used, all of which appeared grossly normal without any obvious forefoot deformity. The feet were deep frozen at -80o C and then thawed for 12 hours before testing. Deep-frozen specimens were deemed suitable for our study since it has been shown that freezing does not affect the biomechanical properties of tendons.19
The hindfoot and the metatarsal of each specimen were transfixed to a wooden block with three Steinmann pins. Two 8-mm pins were placed in the hindfoot and one 4-mm pin was placed in the proximal first metatarsal. The foot was mounted in the lateral position onto a custom aluminum jig attached to an MTS Bionix 858 materials tester (MTS Systems, Inc., Eden Prairie, Minnesota). Initial testing was performed on two amputation specimens to establish the methodology for the study; the data generated from these two specimens were not included in the study results.
The hallux and first metatarsal were oriented parallel to the plate (Fig. 1). Two 1-mm wires were inserted retrograde through the hallux distal phalanx into the proximal phalanx. Changes in the pull of the flexor hallucis longus and flexor hallucis brevis may alter the position of the hallux associated with hallux varus. However, insertion of the longitudinal wires in the described manner would eliminate this potential effect. A third wire, 2 mm in diameter, was inserted through the proximal phalanx from dorsal to plantar in the center of the phalanx in the sagittal plane of the foot, 2 mm distal to the MP joint. This wire was then parallel to the testing plate. A wire loop was attached to the 2-mm wire transfixing the hallux and then attached to the materials tester. Both a vertical translational force and an angular (varus) moment about the hallux MP joint were possible in this model. The vertical force was consistent at 90o to the medial axis of the metatarsal shaft. However, the angular moment about the MP joint depended on how far from the MP joint the wire was inserted. As the distance from the MP joint increased, the angular moment decreased, and vertical translation decreased. During the initial testing, which was performed only on the first two specimens, the wire was placed 2 mm distal to the articular surface of the MP joint and, with the force applied, vertical translation without an angular moment of the MP joint occurred. Since we attempted to create an unstable MP joint and a deformity of hallux varus, the wire had to be inserted more distally on the proximal phalanx. The final position for insertion of the wire was 1 cm distal to the MP joint, and this position was maintained for all the subsequent specimens tested.
The foot was then tested in the above-described position. A preload of 5 N was applied, which had been determined in the preliminary testing to be the required force to place tension on the hallux without causing displacement or angulation. The force was then applied at a constant rate of displacement of 2 mm/min up to a maximum of 4 mm. The first of three load cycles was applied to the intact hallux in the position described above.
After this sequence of testing, a 2.4-cm incision was made in the skin in the first web space. The lateral capsule of the MP joint was incised from the dorsolateral of the joint immediately adjacent to the EHB tendon plantarward to the fibular sesamoid. A similar sequence of testing of the hallux was carried out after incision of the adductor tendon and then after sectioning of the lateral flexor hallucis brevis tendon.
The foot was then removed from the jig and the EHB tendon transfer was performed as follows. The web-space skin incision was extended proximally for 2 inches, and the EHB was transected at the musculotendinous junction (Fig. 2). A 2-0 monofilament suture was inserted into the stump of the tendon using a previously described locking type stitch.13 A large free needle was passed under the deep transverse metatarsal ligament, and the free end of the EHB tendon was brought under the ligament from distal to proximal (Fig. 3). A tunnel was then drilled in the dorsomedial first metatarsal 1.5 cm proximal to the joint and directed plantar, distal, and lateral to exit the metatarsal adjacent to the level of the joint (Fig. 4). The tendon was then passed through the bone tunnel from plantar lateral and distal to dorsal proximal and medial (Fig. 5). A cancellous screw was inserted into the metatarsal 1.5 cm proximal to the bone tunnel to secure the suture under tension. In each specimen, the tension was set on the EHB tendon to position the hallux in 5o of valgus. The range of motion of the hallux MP joint in plantarflexion and dorsiflexion was evaluated before and after the EHB tenodesis using a goniometer. The foot was then remounted and tested similarly.
To ensure that the procedure was reproducible and that the elastic limit of the construct was not passed, load displacement curves were plotted and each measurement was repeated three times for each phase of the testing performed. The mean of these three displacement curves was utilized for final analysis.
Results
The mean force required to displace the intact joint 4 mm was 23.17 N (SD = 4.93 N). Sequential division of the lateral capsule, adductor tendon, and lateral flexor hallucis brevis tendon was then performed. After division of the lateral capsule, the mean force required to displace the hallux 4 mm was 13.93 N (SD = 3.4 N); that after division of the adductor tendon was 7.54 N (SD = 2.97 N); and that after division of the lateral flexor hallucis brevis tendon was 4.25 N (SD = 1.0 N) (Fig. 6).
Division of the lateral capsule therefore caused a 42.2% reduction of the load required to displace the hallux 4 mm, division of the adductor tendon reduced the required force by 67.4%, and division of the lateral flexor brevis tendon reduced the required force by 81.6%. After the EHB tenodesis, the mean force required to displace the hallux 4 mm was 23.28 N (SD = 6.1 N). After the tenodesis, the load displacement curves were equal to those of the intact joint (Fig. 7). Dorsiflexion of the hallux MP joint decreased by a mean of 10o (range, 5 -- 20o) whereas plantarflexion was unaffected after the tenodesis. Comparing a total of five groups (six specimens per group), a repeated measures analysis of variance (ANOVA) demonstrated statistical significance with F = 25.05 (P = 0.05). The hallux was noted to externally rotate (supinate) by approximately 5o in each specimen.
Discussion
Acquired hallux varus is most commonly the result of iatrogenic interruption of the lateral conjoined tendon with over-pull of the abductor hallucis.1,7,12,14 Excision of the lateral sesamoid in conjunction with adductor hallucis release eliminates the normal restraints to medial deviation of the hallux. The development of acquired hallux varus has been attributed to various factors, including removal of the fibular sesamoid, excessive medial capsular reefing, removal of an excessive amount of the medial eminence, over-correction of the intermetatarsal angle, medial tibial sesamoid subluxation, excessive plantar-lateral release, and excessive postoperative bandaging.1,3,10,14,21
In this study, we have attempted to quantify the contribution of the lateral soft-tissue complex to varus dislocation of the hallux. Although we have documented some of these anatomic restraints, we cannot infer each structure's relative contribution since the order of division of each structure was not randomly performed. We therefore cannot state that the adductor tendon contributes more than the lateral flexor brevis tendon to maintaining dynamic balance. Reversing the order of division of these structures might have indicated the relative contribution to lateral restraint. We did not believe, however, that this step was necessary, since we were attempting to create an unstable joint, which occurred after division of the flexor brevis tendon. For the same reason, a sesamoidectomy was not performed.
Hallux varus may be associated with a flexion contracture of the interphalangeal joint as a result of disruption of the flexor brevis tendon. If flexible, hallux varus is not always symptomatic. However, difficulty with shoe wear and pain over the medial and dorsal aspects of the hallux may require surgical correction. Surgical attempts at correction of the deformity with abductor release, or lateral capsular and conjoined tendon repair alone, have proved unsuccessful, presumably because they fail to address the imbalance of forces across the MP joint.6,17 The addition of a tendon transfer to the medial soft-tissue release is recommended in most cases of hallux varus in young, active patients with no arthritis and good range of motion at the MP joint.2,5,8,9,11,22 Dynamic correction of the muscle imbalance with tendon transfers has involved the use of the abductor hallucis or, more commonly, the EHL tendon. Johnson and Spiegl9 recommended a transfer of the entire EHL tendon to the base of the proximal phalanx combined with fusion of the interphalangeal joint. In the absence of deformity at the interphalangeal joint, Johnson8 later recommended splitting the EHL tendon and using the lateral half for correction of the varus deformity. This technique, however, may be difficult if the EHL is scarred and contracted from previous surgery. We have occasionally experienced difficulty with the technique of the EHL transfer as described, and furthermore, have found that both active and passive dorsiflexion of the hallux may be compromised after its use. It is unclear whether the EHL will function as a dynamic tendon transfer and, other than the strength of the EHL, may offer no significant advantage over another static tendon transfer or tenodesis.
As an alternative to the EHL, we propose the EHB tendon as a tenodesis for correction of acquired hallux varus. Because the EHB inserts into the dorsolateral base of the proximal phalanx, it is suited to act as a static restraint to extension and varus forces on the hallux when the transverse metatarsal ligament is used as a pulley. In this study, the EHB clearly corrected the deformity of hallux varus and, by functioning as a static tenodesis, prevented varus dislocation of the hallux to applied load. Since the EHB tendon is passed under the deep transverse metatarsal ligament in performing this procedure, the range of motion of the hallux MP joint is inevitably compromised. In this cadaver study, the mean decrease in motion after the EHB tenodesis was 10o of passive dorsiflexion, whereas plantarlfexion was unaffected. It is unlikely, however, that this decrease in motion will be clinically significant, and in patients where preservation of motion is important, this tenodesis may be a useful alternative to treatment. Although very slight supination of the hallux occurred in this cadaver model, it remains to be seen whether this is clinically significant, since supination of the hallux is unlikely to be symptomatic.
References
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Figure Legends
Fig. 1. Artist's sketch shows the position of the foot and the orientation of the pins in the hallux.
Fig. 2. Artist's sketch shows the dorsal incision and the transection of the EHB tendon.
Fig. 3. The transected tendon is passed deep to the deep transverse metatarsal ligament from distal to proximal.
Fig. 4. A drill hole is made in the dorsomedial first metatarsal.
Fig. 5. The EHB tendon is pulled through the drill hole and secured with sutures to periosteum or bone.
Fig. 6. Graphic representation of the load required to displace the hallux 4 mm in the intact joint, and after incision of the lateral capsule, adductor tendon, and lateral flexor brevis tendon.
Fig. 7. Graphic representation of load displacement curves for each sequential step. I, intact joint; R, repair after tendon transfer; C, lateral capsular incision; A, adductor tenotomy; F, lateral flexor brevis tenotomy.
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