An association between functional second metatarsal length and midfoot arthrosis

Background: Primary tarsometatarsal arthrosis is relatively uncommon. The etiology of osteoarthritis in the foot is poorly understood, and it is possible that mechanical or anatomic factors play a role. Methods: We compared the relative length of the metatarsals in patients with idiopathic arthrosis of the midfoot with that in a group of controls without arthrosis. We analyzed the radiographs of all patients who had had an arthrodesis of the first, second, and third tarsometatarsal joints to treat arthrosis during a three-year period at a tertiary teaching hospital. We excluded patients with a history of inflammatory arthritis, trauma, or Charcot arthropathy. Nine patients (fifteen feet), seven women and two men with an average age of 64.2 years, met the inclusion criteria. We compared them with a control group consisting of the uninjured feet of patients with an acute traumatic injury to the hindfoot and the feet of volunteers with no foot problems. We measured the first, second, and fourth metatarsal lengths and the intermetatarsal angles on weight-bearing anteroposterior radiographs. We also measured the length of the first metatarsal relative to the long axis of the second metatarsal to define the functional first metatarsal length. The ratios of metatarsal lengths and the ratios of functional lengths were used for analysis to minimize differences in foot size and differences caused by radiographic magnification. Statistical comparisons between groups were then carried out. Results: In the study group, the length of the first metatarsal was, on the average, 77.0% of the length of the second metatarsal, whereas, in the control group, the first metatarsal length was an average of 82.0% of the second metatarsal length. The functional length of the second metatarsal was, on the average, 18.6% greater than that of the first metatarsal in the study group and only an average of 4.1% greater than that of the first metatarsal in the control group. Both differences were significant (p < 0.0004 and p < 0.0001, respectively). Conclusions: Patients with midfoot arthrosis had a different ratio of the first to the second metatarsal length than did a similarly aged cohort without midfoot arthrosis. The patients had a relatively short first metatarsal or a relatively long second metatarsal, or both. Midfoot arthrosis may have a mechanical etiology. Recognition of risk factors is the first step in developing prevention strategies.