Deep Transverse Metatarsal Ligament and Static Stability of Lesser Metatarsophalangeal Joints

Abstract

Background:

The static support that guides motion around the lesser metatarsophalangeal joints (MTPJs) is complex. Biomechanical studies revealed important roles of both the plantar plane and collateral ligaments. Since part of the plantar plate is attached to the deep transverse metatarsal ligament (DTML), we hypothesized that the transection of the DTML in the intermetatarsal space may substantially reduce the MTPJ stability.

Methods:

The second, third, and fourth MTPJ stabilities of 6 fresh-frozen human cadaveric foot specimens were measured under load control. Both dorsiflexion and dorsal subluxation conditions were tested. After the intact condition was assessed, the DTML was sequentially transected such that each MTPJ had a unilateral and then a bilateral DTML transection. Stiffness data were calculated using the loading range in each test condition. Paired Student t tests were performed to test for statistical significance (P value less than .05).

Results:

In intact specimens, the mean stiffness with dorsiflexion of the second, third, and fourth toes was 0.52 ± 0.15 N/deg. When the DTML was operatively transected on one side, the dorsiflexion stiffness significantly decreased 17.3% to an average of 0.43 ± 1.00 N/deg (P < .001). Subsequent transection of the DTML on the other side of each joint resulted in a further significant decrease of 5.8% to an average of 0.40 ± 0.08 N/deg (P < .001). The mean stiffness with dorsal subluxation of the intact second, third, and fourth toes was 3.55 ± 0.66 N/mm. When the DTML was operatively transected on one side, the dorsal subluxation stiffness significantly decreased 16.1% to an average of 2.98 ± 0.64 N/mm (P < .001). Subsequent transection of the DTML on the other side of each joint resulted in a further significance decrease of 7.6% to an average of 2.71 ± 0.48 N/mm (P = .016).

Conclusion:

The DTML has a significant role in maintaining lesser MTPJ ligament stability. Both unilateral and bilateral DTML transections caused substantial instability of the lesser MTPJ.

Clinical Relevance:

The DTML is part of the natural static restraint to dorsiflexion or dorsal subluxation of the lesser MTPJ. Operative transection, injury, or degeneration of this ligament may predispose the adjacent MTPJ to instability.

Keywords

lesser metatarsophalangeal joint deep transverse metatarsal ligament plantar plate lesser metatarsophalangeal joint instability

Lesser metatarsophalangeal joint (MTPJ) instability clinically manifests with symptoms including pain, swelling, weakness, and deformity.^8,10,11 Despite the high frequency of this problem, understanding the anatomic basis for this instability has remained somewhat elusive. A partial understanding has come from anatomic and biomechanical studies that revealed the importance of the plantar plate and collateral ligaments.^1,2,4,14

The lesser MTPJ are thought to be primarily stabilized by a combination of static resistance provided by the plantar plate and collateral ligaments and the dynamic pull of the intrinsic flexors.⁹ Elegant dissection work by Deland et al⁵ and Johnston et al⁶ has shown that the plantar plate has a number of important attachments including the collateral ligaments, the plantar fascia, the fibrous sheath of the flexor tendons, the interossei tendons, and deep transverse metatarsal ligament (DTML). Deland et al⁵ described the proximal attachment of the plantar plate to originate from the underside of the metatarsal head. They found this tissue to be “thin” and “fibrous and synovial type on histologic sections.” The thickness of the plantar plate averaged 0.4 mm at the origin but 5 times that (2 mm) throughout the main part of the plate.⁵

In several previous biomechanical studies the plantar plate has been shown to be a major static stabilizer of the lesser MTPJ.^2,4,14 This raises the question of how the plantar plate actually restricts motion, since its putative origin from the underside of the metatarsal heads is synovial and much thinner than the main part of the ligament. Stainsby¹³ found in his anatomic study that the DTML could be a static structural restraint. He described the DTML as attaching to the medial and lateral border of the plantar plates, forming a strong ligamentous structure transversely across the plantar aspect of the forefoot. He hypothesized its role as a “tie-bar” system preventing undue splaying of the forefoot.¹³ This anatomic observation stimulated interest in performing the present study, with the purpose to better understand if the DTML statically restrains motion of adjacent MTPJ, especially since transection of the DTML is performed as part of some common forefoot surgeries.

Materials and Methods

Specimen Preparation

Following an institutionally approved protocol, fresh-frozen human foot specimens (midtibia through toes) from deidentified decedent donors were obtained from nationally recognized tissue donor facilities (Science Care, Phoenix, AZ; United Tissue Network, Phoenix, AZ; Anatomy Gifts Registry, Hanover, MD). All forefeet were grossly normal. Any specimens with evidence of previous surgery or noticeable toe deformity were not included. A total of 6 fresh-frozen human cadaveric foot specimens were utilized for this study. Specimens were from 2 male and 4 female donors, with a reported age at death ranging from 43 to 75 years. All specimens were sealed in airtight plastic bags and maintained in a frost-free freezer (–20°C). Within 24 hours prior to testing, each specimen was in turn, thawed at room temperature (+20°C), still sealed in the airtight plastic bag.

Testing followed closely with our laboratory’s previous work.⁴ Briefly, in attempt to keep the long flexor tendons intact while achieving some uniformity, each specimen was transected 25 cm proximal to the plantar surface of the foot. A Delrin® mounting plate was positioned such that the MTPJ of all toes were located at its edge, allowing unencumbered plantarflexion motion, then secured to the calcaneus using 2-inch drywall screws (GripRite, Irving, TX).⁴

As an interface between each toe and the test equipment, 2 custom fixtures were specifically designed (Sketchup version 8.0, Google, Mountain View, CA) and fabricated from ABS plastic using a rapid prototype machine (Dimension Elite, Dimension, Inc, Eden Prairie, MN), the first for dorsiflexion and plantarflexion motions, the second for dorsal subluxation (Figure 1). Two 0.15 cm holes were drilled laterally through the shaft of the proximal phalanges approximately 1 cm apart. Aluminum wire (18-gauge, Ook Industrial, Pompano Beach, FL) was then threaded through the drilled holes in the bone then into the fixture to attach it to dorsal surface of the toe.⁴

The mounting plate was then secured to a 2.54-cm (1-inch) thick aluminum platform mounted to the load frame of a materials testing system (Model 1331, Instron Corp, Canton, MA) equipped with a ± 250 N electronically calibrated load cell (Model 2527-131, Instron Corp, Canton, MA) and controlled with proprietary control panel and software (FastTRACK 8800, Instron Corp, Canton, MA). Rotation measurements were obtained using a 3D optical tracking system (OptoTRAK 3020, Northern Digital Inc, Waterloo, Ontario, Canada). The OptoTRAK system used infrared light-emitting diodes to represent each rigid-body while the computer calculated vector changes, measuring displacement with an accuracy of 100 microns in the x and y planes and 150 microns in the z plane. This translated to a calculated rotational accuracy of better than 0.05 deg.³ Data were collected using a custom built computer running Windows XP operating system (Microsoft Corp, Redmond, WA) and using LabVIEW 9 (National Instruments, Austin, TX). Stabilizing rods were secured through the grooves in the top of the platform to restrict metatarsal motion during loading of the specimen. These rods rested on the dorsal surface of the metatarsal to prevent vertical motion. To control metatarsal motion, a 20 N force was applied on the upper surface of the rod during positioning. To standardize the test setup, toes were put in anatomical neutral positions using a removable platform extender consisting of a steel plate that supported the rest of the foot. This allowed for reproducible identification of the starting position that simulated the position of the foot and toes in mid stance phase of gait. Once the toes were placed in neutral position, the initial vertical position of the Instron actuator was recorded and rotations for the OptoTRAK system were zeroed.⁴

Biomechanical Testing of the Intact Toes

The second, third, and fourth toes were individually displaced in dorsiflexion/plantarflexion in a continuous cycle using load control to determine the endpoints of rotation. Specimens were displaced with a 25 mm moment arm measured distally from the center of rotation of the lesser MTPJ for dorsiflexion and plantarflexion. For dorsal subluxation motion, the loading device prevented rotation of the toes while a linear displacement was applied in load control perpendicular to the resting plane of the foot.

The 6 feet were divided into 2 groups for testing with 3 feet in each group. In 1 group the second, third, and fourth toes were tested in sequence from medial to lateral in intact condition and then from lateral to medial in disrupted conditions. The other group tested the toes from lateral to medial in intact condition, followed by testing the disrupted conditions from medial to lateral. The testing sequence alternated between dorsiflexion cycling and subluxation cycling to minimize the number of fixture changes per toe. After the 3 toes were tested in the intact condition, the first DTML transection was performed on the opposite side from the next toe to be tested. In this manner, each toe could be tested with both 1-side and 2-side transections with the minimal amount of repositioning of the displacement fixture to minimize variation.

Deep Transverse Metatarsal Ligament Transection

The procedure was performed with the foot mounted on the testing platform to maintain specimen orientation across testing treatments. A dorsal longitudinal incision was made between 2 metatarsal heads. Superficial soft tissue was carefully removed or retracted by blunt dissection. The DTML was exposed and then cut completely (Figure 2). The fat tissue and nerve tissue beneath the DTML were identified to verify complete disruption. The disruption was done by 1 of the 2 orthopaedic surgeons (B.W., O.C.) and was verified by the other.

Motion Cycling—Dorsiflexion

In each test condition, once the displacement apparatus had been affixed to the toe with the toes and foot resting flat on the mounting plate, a 4 N load was applied to preload the specimen and then each toe was cycled from −8 N in plantarflexion to +16 N in dorsiflexion for 30 preconditioning cycles. Then the toe was set back to the 4 N preload value and a final 5 cycles were performed over the same loading range. The data were analyzed for the fifth cycle of the data collection cycles.

Motion Cycling—Dorsal Subluxation

In each test condition, once the displacement apparatus had been affixed to the toe with the toes and foot resting flat on the mounting plate, an 8 N load was applied to preload the specimen and then each toe was cycled from −6 N to +22 N dorsal subluxation for 30 preconditioning cycles. Then each toe was set back to the 8 N preload value and a final 5 cycles were performed over the same loading range. The data were analyzed for the fifth cycle of the data collection cycles.

Data Analyses

Stiffness values were calculated from the loading range and the range of motion of each toe in each test condition. Significance was determined using a predefined P value of less than .05. Data are presented as means ± 95% confidence intervals. Two fourth toes were eliminated from the analysis because they fractured during testing. Data were analyzed using SPSS version 20.0 (SPSS Inc, Chicago, IL) and SigmaPlot version 12.5 (Systat Software Inc, San Jose, CA).

Results

The DTML contributed significantly for controlling sagittal plane motion of the lesser MTPJ, specifically dorsiflexion and plantarflexion (Table 1). The stiffness over the entire range of the cycle from plantarflexion to dorsiflexion of the intact second, third, and fourth toes averaged 0.52 ± 0.15 N/deg. Once the DTML was operatively disrupted on 1 side, the plantarflexion to dorsiflexion stiffness decreased significantly (P < .001) to an average of 0.43 ± 1.00 N/deg, which was a 17.3% decrease. The subsequent disruption of the DTML on the other side significantly decreased stiffness from the intact condition (P < .001) to an average of 0.40 ± 0.08 N/deg, which was a decrease in stiffness of 23.1% relative to the intact condition. The second disruption of the DTML significantly decreased stiffness by 0.03 ± 0.02 N/deg, which was an additional 5.8% decrease in stiffness from the first disruption (P < .001).

Table 1.

Sagittal Motion Stiffness (N/deg) for the Second, Third, and Fourth Toes From 6 Cadaver Specimens.

Specimen	Toe	Intact	First Disruption	Second Disruption
1	2	0.41	0.33	0.30
	3	0.40	0.36	0.32
	4	0.40	0.37	0.33
2	2	1.15	0.84	0.76
	3	1.27	0.95	0.83
	4	FX^a	FX^a	FX^a
3	2	0.37	0.36	0.35
	3	0.36	0.32	0.32
	4	0.41	0.36	0.34
4	2	0.37	0.33	0.32
	3	0.36	0.36	0.35
	4	FX^a	FX^a	FX^a
5	2	0.44	0.42	0.40
	3	0.46	0.42	0.39
	4	0.53	0.42	0.40
6	2	0.40	0.37	0.36
	3	0.47	0.39	0.38
	4	0.51	0.33	0.30
Mean ± 95% CI		0.52 ± 0.15	0.43 ± 1.00	0.40 ± 0.08

Specimen was excluded because fracture occurred during testing.

The DTML also contributed significantly for controlling dorsal subluxation motion of the lesser MTPJ (Table 2). The stiffness over the entire loading range in dorsal subluxation of the intact second, third, and fourth toes averaged 3.55 ± 0.66 N/mm. Once the DTML was operatively disrupted on 1 side, the dorsal subluxation stiffness significantly decreased (P < .001) to an average of 2.98 ± 0.64 N/mm, which was a 16.1% decrease. The subsequent disruption of the DTML on the other side significantly decreased stiffness from the intact condition (P < .001) to an average of 2.71 ± 0.48 N/mm, which was a 23.7% decrease relative to intact. The second DTML disruption significantly decreased stiffness relative to the single sided disruption of the DTML (P = .016) by an average of 0.27 ± 0.26 N/mm, which was a 7.6% decrease relative to the intact stiffness.

Table 2.

Subluxation Stiffness (N/deg) for the Second, Third, and Fourth Toes From 6 Cadaver Specimens.

Specimen	Toe	Intact	First Disruption	Second Disruption
1	2	3.09	2.71	2.24
	3	2.81	2.00	1.78
	4	2.45	2.25	1.83
2	2	3.24	2.33	2.57
	3	3.22	2.52	2.29
	4	FX^a	FX^a	FX^a
3	2	3.12	3.06	2.85
	3	4.07	3.19	2.98
	4	2.46	2.03	3.01
4	2	2.73	2.37	2.30
	3	2.42	2.02	1.82
	4	FX^a	FX^a	FX^a
5	2	5.16	4.96	3.92
	3	6.69	6.05	4.82
	4	5.70	4.73	4.15
6	2	3.31	2.84	2.59
	3	3.10	2.17	1.92
	4	3.26	2.37	2.21
Mean ± 95% CI		3.55 ± 0.66	2.98 ± 0.64	2.71 ± 0.48

Specimen was excluded because fracture occurred during testing.

Discussion

Transection of the DTML is performed as part of some common forefoot surgeries like Morton’s intermetatarsal neuroma decompression or excision and lateral soft tissue release for correction of hallux valgus. The mechanical effect on the forefoot if the DTML is transected was the focus of this cadaveric study. We found transecting the DTML had a reproducible and significant effect on static restraint of lesser MTPJ. This effect was consistent for both dorsiflexion and dorsal subluxation testing. The magnitudes of these effects on an individual MTPJ for unilateral and bilateral DTML transections were remarkably similar. When the DTML was operatively transected on 1 side, the dorsiflexion stiffness decreased an average of 17% and the dorsal subluxation stiffness decreased an average of 16%. When DTML was transected on both sides of a joint, the dorsiflexion stiffness decreased an average of 23% and the dorsal subluxation stiffness decreased an average of 24%.

Several previous cadaver based studies have reported the effects of cutting ligaments around the lesser MTPJ on the stability of the joints.^1,2,4,14 In a seminal article by Bhatia et al,² who stapled the FDL tendon to the talus and removed contiguous toes, the plantar plate was measured to contribute 30%, and the collateral ligaments to contribute 46% to vertical translation stability. In this study the FDL tendon was stapled to the talus, and the contiguous toes were removed.² Further studies have generally confirmed these findings—all also using displacement control as the testing modality. Using isolated MTPJ preparations, Suero et al¹⁴ reported the contributions to dorsal translation stability to average 19% for the plantar plate, and 37% for both collateral ligaments. Employing an isolated MTPJ preparation, Barg et al¹ reported that complete release of the accessory and proper collateral ligaments resulted in an average 45% decrease in required load to translate the toe dorsally 3 mm on the metatarsal head. Using an intact whole foot and ankle preparation similar to that used in the present study, Chalayon et al⁴ found the plantar plate contributed an average of 23% and 34% of the tested stability in dorsiflexion and dorsal subluxation, respectively. The magnitude of all these findings, especially for the plantar plate, is consistent with that of the present study, which showed bilateral DTML transection inducing a 23% decrease in dorsiflexion stiffness of the lesser MTPJ.

The main limitation of the present study is the lack of physiological dynamic loading. Many tendons cross the lesser MTPJ, each with specific and important functions. The intrinsic muscles all support plantarflexion the lesser MTPJ⁷ and therefore when active may serve to stabilize the MTPJ. It is possible that this would have some balancing effect on the findings of our study, although the transection may reduce the mechanical advantage of the lumbrical musculature to plantarflex the toes and any dorsal instability may similarly reduce or even reverse the direction of pull of the interossei. The second limitation of the study was the inability to model healing after ligament transection. The effect we see may be only temporary and in a clinical setting may not sustain. Third, the strength of cadaver periarticular structures may not be maintained following freezing and thawing of the tissue, thus could increase joint laxity when compared to that of patients.¹²

One major advantage of our approach was the use of a whole foot and ankle specimens. The use of a whole intact foot and ankle, with all preserved extrinsic tendons, allowed us to move unimpeded by the preparation closely simulated the natural condition. The previous biomechanical studies by Barg et al¹ and Suero et al¹⁴ used the isolated MTPJ specimens with transected DTML on both side. Therefore the results of these studies should be interpreted carefully in terms of the magnitude of the stabilizing effect of the collateral ligaments.^1,14

All specimens were tested under load control and not displacement control. The use of load control allowed us to more closely test the extremes of the restraint envelope. In load control, structures can be subjected to more physiological loads, and any instability can be measured by the magnitude of the motion. In displacement control, an arbitrary displacement must be selected, and resulting loads are measured, which may be more or less than physiological loading conditions. In addition, and critically important to a testing situation where the properties of the stabilizing structures are changing, load control improves the paired comparison of subsequent conditions on each toe since consistent loads are applied across all toes and all treatment conditions. By keeping the loads constant at the endpoints of each motion, it enables the comparison of the dependent variable, displacement, to be more meaningful. The test system used automatically adjusted the mean position of the cycle and the magnitude of the displacements to compensate for any biomechanical changes in the specimens caused by our treatment conditions. Under load control, the system was also automatically adjusting for stretching which occurred with additional cycling. Finally, in this study, we tracked motion with a high precision optical tracking system with a theoretical accuracy of 100-150 microns. This permitted us to generate accurate load displacement curves and subsequently calculate stiffness values.

Conclusions

In this cadaver-based study, we found the DTML to be a critical static stabilizer of the lesser MTPJ. Transection of this structure in a laboratory setting resulted in significant MTPJ instability. Surgeons should be aware that cutting the DTML may result in adjacent lesser MTPJ instability.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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