Biomechanical consequences of first metatarsaophalangeal joint arthrodesis on flexor digitorum longus function: A cadaveric study

Abstract

Background:

First metatarsophalangeal joint (MTPJ) arthrodesis is a common treatment modality for hallux rigidus with successful outcomes. However, the effect of arthrodesis on flexor digitorum longus (FDL) is poorly understood. The purpose of this study was to investigate this effect in a biomechanical model.

Methods:

Ten cadaveric trans-knee amputated specimens were studied. Lesser toe range of motion (ROM) and FDL excursion on simulated FDL contraction were measured in the following three scenarios: (1) before 1st MTPJ arthrodesis, (2) after 1st MTPJ arthrodesis, and (3) after the knot of Henry release.

Results:

1st MTPJ arthrodesis reduced both mean lesser toe ROM and FDL excursion. However, there was improvement in these parameters after the knot of Henry release.

Conclusions:

FDL function was reduced following 1st MTPJ arthrodesis. This effect was contributed by soft tissue connections at the knot of Henry, where FDL crosses flexor hallucis longus (FHL). Therefore, with restriction of FHL movement after 1st MTPJ arthrodesis, FDL movement was also restricted.

Clinical Relevance:

This result improved our understanding of 1st MTPJ arthrodesis biomechanics and suggested potential benefits of the knot of Henry release in 1st MTPJ arthrodesis to improve FDL function, thereby decreasing the rate of metatarsalgia and other associated problems.

Introduction

Hallux rigidus is a painful condition of the 1st metatarsophalangeal joint (MTPJ), with restricted motion and osteoarthritic changes of the joint. It is the second most common condition of the 1st MTPJ after hallux valgus. ¹ This condition commonly occurs in the elderly population, ² though adolescent form of hallux valgus had been described. ³ It can affect the daily functions of the patients in various aspects especially in sports activities. Even though nonoperative treatment is the initial treatment in most cases, surgery is an option when it fails.

There are different surgical treatment options ^{4 –6} for hallux rigidus, with arthrodesis being one of the options with good outcome. Results of 1st MTPJ arthrodesis had been historically good with union rate as high as 100%, ⁷ and significant improvement of patient satisfaction up to 85%. ^8,9 Various methods of joint preparation had been described, such as flat cuts and ball and socket. ¹⁰ There were also several methods of fixations described, such as dorsal plate and compression screws, which had been shown to be the strongest biomechanically. ^{11 –13} This procedure is commonly done by open techniques, but arthroscopic technique had been described. ¹⁴

This procedure is however not without complications. Nonunion, malalignment, interphalangeal joint pain, and delayed union are potential complications from this procedure. Lesser toe metatarsalgia, defined as pain in either the heads of the metatarsals or the MTPJs, is one of the most common problems encountered by patients after arthrodesis. In a multicenter retrospective study by Kim et al., ¹⁵ metatarsalgia incidence can be as high as 9.8% in patients after arthrodesis. Long-term review of 100 cases of 1st MTPJ arthrodesis by Fitzgerald et al. ¹⁶ showed that 10% of patients either developed metatarsalgia or had worse symptoms of metatarsalgia after operation. However, the mechanism of lesser toe metatarsalgia after 1st MTPJ arthrodesis is poorly understood in the current literature.

Metatarsalgia is caused by abnormal forefoot loading. Several biomechanical studies ^17,18 have shown that the loss of FDL function causes significant change of forefoot loading from lesser toes to metatarsal heads. With this in mind, we aim to investigate the hypothesis that the 1st MTPJ arthrodesis leads to loss of FDL function and therefore causes abnormal forefoot loading and metatarsalgia. However, there is no current literature that shows evidence of loss of FDL function in patients with 1st MTPJ arthrodesis.

We postulate that the loss of FDL function in 1st MTPJ arthrodesis is primarily due to the soft tissue connection at the knot of Henry, where flexor digitorum longus (FDL) crosses flexor hallucis longus (FHL). Therefore, with restriction of FHL movement after 1st MTPJ arthrodesis, excursion of FDL and therefore the active movement of lesser toes will be affected.

The goals of this study are to investigate the effect of 1st MTPJ arthrodesis on FDL function and study the role of the knot of Henry as a contributing factor to this effect. With the paucity of data in this area, this study will improve our understanding of foot biomechanics in 1st MTPJ arthrodesis and may potentially offer a solution in preventing lesser toe metatarsalgia in this group of patients.

Materials and methods

Ten fresh frozen cadaveric specimens from different donors were used. All the specimens were non-embalmed and displayed no deformities. Specimens used were trans-knee amputated specimens. Dissection was carried out at the medial malleolus to identify the musculotendinous junction of FDL. Another area of dissection was carried out at plantar aspect of forefoot to identify FDL of the 3rd toe for the purpose of tendon excursion measurement.

Proximal tensile force measured 50 N was then exerted by a single investigator at FDL musculotendinous junction to simulate active plantar flexion of the lesser toes. Third toe range of motion (ROM) was used as the reference to lesser toe ROM in general in view of its central location in the foot. Third toe MTPJ and proximal interphalangeal joint (PIPJ) flexion ROM were measured visually using goniometer with reference to metatarsals, with ankle in the stationary position at about 20° of plantar flexion. Third toe FDL excursion was measured using a fixed reference point of Kirschner (K)-wire inserted perpendicular to the plane of the foot from plantar aspect. Where the K-wire then runs perpendicular to the tendons of the FDL, a surgical marker was used to indicate the corresponding level on the FDL with the foot in the neutral position. On application of proximal tensile force at the FDL musculotendinous junction, the difference between the reference point and the surgical marker on the FDL was taken as the excursion of FDL.

First MTPJ arthrodesis was performed by two crossed 1.6-mm K-wires (Figure 1) in the arthrodesis angles of 20–25° of dorsiflexion ¹⁹ and 10–15° of valgus. K-wire position was checked with fluoroscopy (Figure 2). No joint exposure and preparation of the 1st MTPJ was performed.

Figure 1.

First MTPJ arthrodesis with two crossed wires. MTPJ: metatarsophalangeal joint.

Figure 2.

Fluoroscopy of first MTPJ arthrodesis. MTPJ: metatarsophalangeal joint.

Dissection over the plantar and medial aspect of midfoot was performed to identify the knot of Henry. Complete release of soft tissue connection between FDL and FHL (Figure 3) was performed to isolate both tendons. Proximal tensile force was again exerted at the FDL musculotendinous junction to simulate active plantar flexion of the lesser toes. Third toe flexion ROM and 3rd toe FDL excursion post 1st MTPJ arthrodesis were measured.

Figure 3.

Knot of Henry.

Mean values of 3rd toe flexion ROM and FDL excursion pre and post 1st MTPJ arthrodesis were compared using paired Student’s t-test. Statistical significance was defined as p value <0.05.

Results

Of the 10 cadaveric specimens used in the study, there were 5 left lower limbs and 5 right lower limbs. The average age of the donors was 78 (range 73–89) years.

There was significant decrease in the mean 3rd toe flexion ROM and FDL excursion post 1st MTPJ arthrodesis, with substantial improvement after the knot of Henry release. Before 1st MTPJ arthrodesis, mean 3rd toe MTPJ ROM and PIPJ ROM were 20° and 22°, respectively. After 1st MTPJ arthrodesis, these values decreased to 9.5° (p = 0.002) and 14° (p = 0.002), respectively, and after the knot of Henry release, these values increased to 16.5° (p = 0.002) and 19° (p = 0.020), respectively. Before 1st MTPJ arthrodesis, mean 3rd toe FDL excursion was 6 mm. After 1st MTPJ arthrodesis, this value decreased to 4.7 mm (p = 0.003) and after the knot of Henry release, this value increased to 5 mm (p = 0.041). The results are summarized in Table 1.

Table 1.

Outcomes at different steps: Pre 1st MTPJ arthrodesis, Post 1st MTPJ arthrodesis and Post knot of Henry release.

	Pre 1st MTPJ arthrodesis	Post 1st MTPJ arthrodesis	Post knot of Henry release
Mean 3rd toe MTPJ ROM (degrees)	20	9.5 (p = 0.002)	16.5 (p = 0.002)
Mean 3rd toe PIPJ ROM (degrees)	22	14 (p = 0.002)	19 (p = 0.020)
Mean 3rd toe FDL excursion (mm)	6	4.7 (p = 0.003)	5 (p = 0.041)

MTPJ: metatarsophalangeal joint; PIPJ: proximal interphalangeal joint; ROM: range of motion; FDL: flexor digitorum longus.

Discussion

The present study has shown significant decrease in FDL function post 1st MTPJ arthrodesis as evidenced by the decrease in the 3rd toe flexion ROM and FDL excursion. There was significant improvement after the knot of Henry release, which showed that soft tissue connection at the knot of Henry was likely to be the major contributory cause of FDL function loss in 1st MTPJ arthrodesis.

Active FDL function has an important role in balancing load between lesser toes and metatarsals during ambulation. Numerous studies had shown the importance of FDL in reducing forefoot loading over the metatarsals. In a cadaveric study by Ferris et al. ¹⁷ using pneumatic muscle actuators and pedobarographic measurement of forefoot loading, the authors found that loss of both FHL and FDL activity caused significant decrease in contact area and pressure beneath the toes and increase in contact area and pressure under the forefoot metatarsals.

In another similar study by Hamel et al. ¹⁸ , it was found that in the absence of FDL function, the contact area and force shifted from underneath the toes to that of the metatarsal heads. With increased contact area and force underneath the metatarsal heads, the likelihood of developing metatarsalgia is higher. Therefore, loss of FDL function in 1st MTPJ arthrodesis, as shown by our study, is likely to cause metatarsalgia clinically.

Besides metatarsalgia, there are also other complications associated with the loss of FDL function. Firzgerald et al. ¹⁶ had noticed MTPJ subluxation and overlying callosities to be complications in 1st MTPJ arthrodesis. These conditions are also related to forefoot loading in the FDL-deficient foot, which has been shown in our study to be present in 1st MTPJ arthrodesis.

Lesser toe plantar flexion is also important in body balancing in both standing posture and gait. Loss of lesser toe plantar flexion strength will thus lead to loss of balancing ability. ^20,21 Mickle et al. ²² had shown that decreased toe plantar flexion strength may even lead to increased incidence of falls in the older population. Exercises to strengthen toe plantar flexion had also been described to improve standing balance in older population. ²³ Similarly, in 1st MTPJ arthrodesis, loss of FDL function with subsequent decrease in lesser toe plantar flexion will cause problem of balancing in patients after 1st MTPJ arthrodesis. Ways to improve FDL function will therefore reduce the risk of falls, which can potentially lead to other complications such as implant breakage.

Knot of Henry was first described by Henry ²⁴ as tying of tendons of FHL and FDL in the inferior aspect of the summit of the vault of the foot. O’Sullivan et al. ²⁵ had described the different patterns of tendinous slips at the knot of Henry, depending on the direction of the tendinous slips, either toward FHL from FDL or toward FDL from FHL. In the same study, it was also found that depending on the patterns of tendinous slips, traction on one of the tendons, either FHL or FDL, caused movement of the toes controlled by another tendon. These findings signified the presence of not just the anatomical relationship but also the biomechanical interaction between FHL and FDL.

Our study successfully demonstrates that the knot of Henry is the major contributory cause of FDL function loss in 1st MTPJ arthrodesis. This finding is clinically important as it demonstrates the relevance of releasing the knot of Henry during 1st MTPJ arthrodesis to reduce forefoot loading, with subsequent reduced rate of metatarsalgia, MTPJ subluxation, overlying callosities, and falls in the elderly population. This procedure is safe in clinical practice as there are no important neurovascular structures at this area of dissection. Most surgeons are familiar with this dissection as it involves a similar area with that of tendon transfer in flexible flat foot deformity, which is a common procedure for most foot and ankle surgeons. ²⁶

In the present study, there is no full recovery of FDL function after the knot of Henry release, indicating the presence of other less significant contributory causes. We postulate the role of plantar fascia in the limitation of lesser toes movement. Plantar fascia has a common origin from calcaneum with subsequent branching into five toes. Restriction of 1st MTPJ movement will therefore limit MTPJ movement of lesser toes. This hypothesis is however not tested in our study and further biomechanical studies are needed to confirm this hypothesis.

There are several limitations of our study. The experiment was performed and data were collected by a single investigator, and this limitation increases the risk of investigator bias. In the present study, crossed K-wires with lower biomechanical stability were used for 1st MTPJ arthrodesis because of cost constraints. However, this does not affect the outcome of the study as any potential micromotion at the 1st MTPJ is unlikely to cause significant FHL excursion to affect the results.

Reference point for FDL excursion measurement was a K-wire fixed to the underlying metatarsal bone. The K-wire may restrict the movement of deeper soft tissue and the underlying intrinsic muscles. However, we do not think this is significant, as comparison of tendon excursions was made pre and post 1st MTPJ arthrodesis, where the same reference point was used, therefore eliminating any potential bias.

We also used goniometer for measurement of ROM. Use of goniometer is subjective and is therefore less accurate. However, we tried to compensate for this limitation by including tendon excursion distance, which serves as another measurement for outcome.

This is the first study in the current literature to show the loss of FDL function in patients with 1st MTPJ arthrodesis. We have also proved that the knot of Henry is the major contributory cause of this effect. This is a major breakthrough as exploration and release of knot of Henry as part of 1st MTPJ arthrodesis in clinical practice may prevent loss of FDL function and therefore reduces the incidence of metatarsalgia and other associated complications of forefoot loading.