First Metatarsal Pathology: Can An Implant Provide A Long-Term Solution?

Author(s): 
By Kerry Zang, DPM, Shahram Askari, DPM, A’Nedra Fuller, DPM, and Chris Seuferling, DPM

Addressing the biomechanics of the first metatarsophalangeal joint (MPJ) as well as the first ray are the keys to any surgical correction of first metatarsal pathology. According to Rootian theory, the principal etiologies of hallux limitus are as follows.1
A long first metatarsal or when the position of the first metatarsal head is relative to the second. When the first metatarsal is long, there will be jamming of the metatarsophalangeal joint during the initiation of the propulsive phase of gait. This causes a reduction in the range of dorsiflexion of the hallux and increases the ground reactive forces in the joint, resulting in early arthritic joint changes.2,3
Hypermobility of the first ray. This occurs when pronation of the subtalarjoint removes the mechanical advantage of the peroneous longus tendon on the first metatarsal, unlocking it and allowing dorsiflexion through midstance and propulsion. This leads to improper articulation at the first metatarsophalangeal joint and subsequent arthritic changes.1
Metatarsus primus elevatus. Metatarsus primus elevatus, a dorsally positioned first metatarsal relative to the lesser metatarsals, causes destruction of the joint, similar to hypermobility.
An immobilized first ray. Either bony ankylosis of the first metatarsocuneiform joint or congenital coalition may cause immobility of the first metatarsophalangeal joint. This causes the hallux to accept part or all of the normal first metatarsocuneiform joint motion in its articulation with the first metatarsal.1
Arthritic joint changes and trauma. Generalized degenerative joint disease— whether it is traumatic in origin or brought on by a multitude of other causes (hallux valgus, systemic arthritidies-rheumatoid) — will also stress the range of hallux dorsiflexion during gait. Generalized degenerative joint disease is usually the presenting clinical/symptomatic diagnosis for the patient with hallux limitus, regardless of the biomechanical etiology.
Although not a primary etiology of hallux limitus, the presence and degree of metatarsus primus adductus requires the utmost attention in order to achieve surgical success. Determining the nature of the articulation of the first metatarsocuneiform joint and whether this joint is stable can further affect the overall outcome of a total first metatarsophalangeal joint arthroplasty. In conjunction with resurfacing the diseased articular cartilage, it is necessary to correct any structural abnormalities present. If the joint is unstable, a repositional arthrodesis of the first metatarsocuneiform joint or a repositional osteotomy of the first metatarsal may be required for a successful outcome.

Why First Metatarsophalangeal Joint Implants Fail
Over the years, there have been many first metatarsophalangeal joint implants, hemi and total, that have attempted to resolve the aforementioned pathologies (see “An Overview Of Previous First Metatarsophalangeal Implants” below). Unfortunately, most of these implants have failed to provide long-term relief of symptoms.

There are a variety of reasons for these implant failures. Very few constrained (single component) silicone-based implants — and even some implants made of alloy materials — are capable of withstanding the forces transmitted through the first metatarsophalangeal joint. When this occurs, implant destruction or osseous degeneration about the implant follows.
Joint biomechanics are another issue. The natural anatomy of the first MPJ allows for specific fluidities of motion for given levels of activity. Any implant must likewise adapt for those varying activity levels or the implant will fail.
The complexity of the surgical procedure also factors into the equation. Many first MPJ implant procedures are technically complicated and surgeons may be less likely to choose a total joint replacement when a joint destructive procedure, although less gratifying to the patient, will require less operative time and less potential postoperative complications.

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