The Hallux Metatarsophalangeal Capsule: An Anatomic Study With Respect to Percutaneous Hallux Valgus Correction

Methods

Ten nonpaired fresh-frozen cadaveric specimens were used for the study. These specimens were obtained through the Maryland State Anatomy Board. Specimens were excluded if they demonstrated severe pathology (eg, gangrene) or prior surgery about the hallux or medial forefoot. Study cadavers included 5 female and 5 male specimens, with a mean age of 77.4 years (range, 56-101 years). Specimens were thawed at room temperature for dissection. A dorsal approach was performed to each MTP joint. The extensor tendons were transected at the level of the MTP joint to expose the underlying capsule (Figure 1A). The MTP capsule, collateral ligaments, and plantar plate were released off the proximal phalanx (Figure 1B). The toes were then disarticulated through this plane, leaving the capsule attached proximally to the metatarsal (Figure 1C). Next, the capsule was longitudinally split into quarters, which each leaf reflected at the 12-, 3-, 6-, and 9-o’clock positions. Care was taken to leave the proximal attachment of the capsule undisturbed. Medial and lateral sesamoids were not disrupted and left attached to the plantar capsule. The central point of the metatarsal head cartilage was identified. This location was identified as the distalmost point of the metatarsal head. A digital caliper was used to measure the distance from the central point of the metatarsal head cartilage to the most proximal extent of the capsule dorsally, medially, laterally, plantarlaterally, and plantarmedially (Figure 1D). The measurement was taken from an orthogonal plane of the central point to these 5 anatomic capsular origins, parallel to the longitudinal axis of the first metatarsal. Three fellowship-trained board-certified orthopaedic foot and ankle surgeons measured each metatarsal independently. Measurements were averaged among the 3 observers, and intraobserver reliability was calculated.

OPEN IN VIEWER

Figure 1.

A dorsal dissection of the hallux was performed. (A) Skin was excised on the dorsum of the hallux metatarsophalangeal (MTP) joint. (B) The proximal phalanx was then disarticulated by releasing the insertion of the MTP capsule. (C) The medial (M), lateral (L), dorsal (D), and plantar (P) capsular tissue and its relationship to the articular surface of the metatarsal head are demonstrated. (D) After disarticulation of the hallux, a digital caliper was used to measure the distance from the central point of the metatarsal head to the respective capsular origin, parallel to the longitudinal axis of the first metatarsal.

Results

The mean measured distances from the central metatarsal head to the proximal aspect of the dorsal, medial, lateral, plantarmedial, and plantarlateral MTP joint capsule are listed in Table 1 for each respective metatarsal. These mean values are averaged among the independently measured values by each of the 3 senior surgeons. Intraobserver reliability was calculated among the 3 surgeons, with excellent correlation. R was 0.88 for all pairs.

OPEN IN VIEWER

Table 1.

Demographics and Measurements (From the Central Point of the Metatarsal Head to the Respective Capsular Origin) for Each Cadaveric Specimen.

Specimen No.	Sex	Age, y	Dorsal, mm	Plantarmedial, mm	Plantarlateral, mm	Medial, mm	Lateral, mm
1	F	101	14.5	21.4	21.9	8.9	9.7
2	M	69	14.3	15.4	18.9	7.8	8.6
3	F	89	14.1	22.4	24.1	12.7	12.0
4	F	85	14.4	13.9	17.3	6.4	9.3
5	M	57	17.4	22.4	21.7	5.7	10.0
6	M	74	14.7	20.4	21.1	9.5	11.3
7	F	78	13.9	18.7	21.1	6.1	7.8
8	M	91	15.0	18.3	21.0	10.0	9.5
9	M	56	18.9	19.6	20.4	6.9	7.5
10	F	74	14.6	20.5	22.4	10.3	10.4

The mean specimen age and mean measured distance in each dimension are listed in Table 2, with 95% CIs. As shown, the dorsal and plantar limbs of the capsule had more proximal extensions than the medial and lateral limbs.

OPEN IN VIEWER

Table 2.

Mean Age, Mean Distances From the Metatarsal Head, and Respective Standard Deviation/95% CI.

Characteristic	Mean	SD	95% CI
Age, y	77.4	14.5	48.9-105.9
Dorsal, mm	15.2	1.6	12.0-18.4
Plantarmedial, mm	19.3	2.8	13.8-24.8
Plantarlateral, mm	21.0	1.9	17.3-24.6
Medial, mm	8.4	2.2	4.0-12.8
Lateral, mm	9.6	1.4	6.8-12.4

Discussion

Whether performed with traditional open or minimally invasive techniques, the surgical correction of a hallux valgus deformity has similar principles; the deformity is usually corrected with a combination of bony realignment via osteotomy or arthrodesis, along with soft tissue rebalancing. In many areas of orthopaedics, there has been an interest in minimally invasive surgery, from both patients and surgeons alike. This has led to the development of minimally invasive surgical techniques in the treatment of hallux valgus. The first iterations of minimally invasive surgery (MIS) for hallux valgus were introduced by Wilson ¹⁸ and Bösch et al ² in the early 1980s. These were soon followed by Isham, ⁹ who developed an MIS modification of the Reverdin osteotomy and was the first to use the Shannon burr in this application. This burr is unique in that it has both side- and end-cutting flutes. Isham’s technique was combined with a minimally invasive Akin osteotomy, medial eminence resection, and adductor release and used a strict protocol of postoperative dressings in lieu of internal fixation. This procedure showed good short- and long-term outcomes but often resulted in an average of 5 mm of shortening. ¹⁰ The risk of shortening and the lack of stable internal fixation prevented many orthopaedists from adopting this early technique.

The next generation of MIS techniques for hallux valgus correction was introduced in the 1990s. Magnan et al ¹⁴ and Giannini et al ⁶ pioneered the SERI procedure (Simple, Effective, Rapid, Inexpensive). This technique used a vertically oriented osteotomy at the first metatarsal neck, which was then splinted by a Kirshner wire that was passed into the medullary canal proximally. Unfortunately, this technique was not found to be reproducible.^7,8 Kadakia et al ¹¹ showed poor results with significant complications, such as dorsal malunions, early recurrence, avascular necrosis of the distal segment, and wound complications. Due to these risks, the procedure has mostly been abandoned in the modern orthopaedic era.

Vernois and Redfern ¹⁷ developed the minimally invasive chevron/Akin bunionectomy procedure in the 2000s. Their technique uses a percutaneous distal first metatarsal chevron osteotomy and a percutaneous proximal phalanx Akin osteotomy. This MIS osteotomy is likely attractive to surgeons due to their familiarity with the open chevron osteotomy, as well as its adherence to the principle of bony fixation with screws. The described technique of this osteotomy begins with a 3-mm stab incision at the first metatarsal metaphyseal–diaphyseal junction, followed by using a periosteal elevator to elevate the soft tissue in the path of the osteotomy. Next, a 2-mm × 20-mm Shannon burr is used to create the apex of the chevron, followed by the dorsal and plantar limbs of the osteotomy with the side-cutting flutes of the burr. As orthopaedists adopt this minimally invasive technique, they would benefit from understanding the surrounding capsular anatomy. Previous studies have described the anatomy of the neural and tendinous structures in relation to this osteotomy. Dhukaram et al ⁴ and Malagelada et al ¹⁵ performed cadaveric studies that demonstrated the proximity of the dorsomedial and dorsolateral cutaneous nerves and extensor hallucis longus tendon in relation to the MIS techniques. They showed that current MIS techniques are safe when it comes to risk of damage to the local structures. The goal of our study was to evaluate the anatomy of the first metatarsal hallux MTP capsule in relation to the MIS chevron osteotomy.

With percutaneous osteotomies of the first metatarsal for the treatment of hallux valgus becoming more popular, it is of paramount importance to understand the capsular anatomy of the MTP joint. Lucas y Hernandez et al ¹³ have advocated for an extra-articular cut with a percutaneous extra-articular reverse-L chevron (PERC) osteotomy. The authors believe that an extra-articular osteotomy can potentially hold benefits compared to an intra-articular osteotomy in 2 ways. Limiting capsular trauma may decrease the pain and stiffness after surgery. Furthermore, any osteotomy created with a burr or saw inherently creates bony debris. Such debris that remains in the joint even after a thorough irrigation can possibly lead to postoperative pain, stiffness, and third-body wear.

Our study showed the shortest mean capsular distance on the medial and lateral portions, with 8.4 mm (4.0-12.8) and 9.6 mm (6.8-12.4) respectively. Dorsally, the capsule originated a mean of 15.2 mm (12.0-18.4) from the distal center of the metatarsal head. The plantar capsule inserted proximally the farthest from the metatarsal head, 19.3 mm (13.8-24.8) plantarmedially and 21.0 mm (17.3-24.6) plantarlaterally (Table 2). Intuitively, this correlates with the range of motion in the respective planes. Typically, there is more dorsiflexion compared to the amount of plantarflexion in the hallux MTP joint. Similarly, there is much less varus/valgus motion than sagittal motion, correlating with the more distal capsular insertions medially and laterally. Using the results from our study, we have derived the following guidelines for surgeons who would like their osteotomy to be extra-articular (using 95% CIs and rounding to the nearest millimeter): apex of osteotomy 12.8 mm from the distal extent of the metatarsal head (Figure 2A), 18.4 mm exiting dorsally for the dorsal limb (Figure 2B), and 24.8 mm plantarly (Figure 2C). Figure 2D shows our proposed osteotomy superimposed on a lateral view radiograph. If surgeons wish to use an angled apex for their osteotomy, either in a plantarward direction to lower the metatarsal head or toward the second metatarsal head to gain length, they can also use these measurements to help guide their cut.

OPEN IN VIEWER

Figure 2.

Based on the findings of our study, we recommend that surgeons who desire an extra-articular chevron osteotomy (A) keep the apex of their osteotomy at least 12.8 mm from the central metatarsal head, and the 2 limbs should extend to at least (B) 18.4 mm dorsally and (C) 25 mm plantarly. (D) A representative radiograph depicting our proposed osteotomy is depicted.

To our knowledge, this is the first study to examine the 3-dimensional anatomy of the MTP capsule of the first metatarsal. By understanding the capsular anatomy, surgeons who wish to use a minimally invasive chevron osteotomy to treat hallux valgus can more precisely localize the osteotomy to ensure extracapsular positioning. It is hypothesized that careful creation of the percutaneous osteotomy limbs with respect to the MTP capsule may yield improved results with fewer complications, although that is beyond the scope of this anatomic study and requires further clinical research.

There are limitations to our study, as with all anatomic cadaver studies. The 10 specimens in our study may not accurately reflect the capsular anatomy of a patient undergoing percutaneous bunion correction, as our specimens did not have obvious hallux valgus deformities. It is unknown if the capsular origins differ between hallux MTP joints with normal alignment vs those with a hallux valgus deformity. In addition, the average age of our specimens was 77 years; the typical patient seeking percutaneous bunion surgery is likely to be younger than this age. It is also unknown if the capsular anatomy changes with aging. Also, cadaveric tissue characteristics are likely different than in the in vivo setting.

The final limitation to our study is the scalability of the findings. We did not measure the specimens, so it is unclear if this can be generalized to all patients. Our study was conducted in the United States, and foot size may be different in other geographic regions. The findings presented in this article are guidelines that surgeons can use to assist while planning an osteotomy but may need to be adjusted with appropriate clinical judgment based on patient-specific anatomy.

Percutaneous hallux valgus surgery continues to be an evolving field. Understanding the capsular anatomy is critical for surgeons to precisely perform these procedures. If surgeons wish to keep their distal metatarsal osteotomy extra-articular, they should plan to keep the apex of their chevron at least 13 mm from the central metatarsal head, and the 2 limbs should extend to at least 18 mm dorsally and 25 mm plantarly. Future studies should focus on clinical and radiographic outcomes as well as comparing the effects of intracapsular vs extracapsular osteotomies.

Supplemental Material