Second Metatarsophalangeal Joint Instability and Second Metatarsal Length Association Depends on the Method of Measurement

Introduction: Second metatarsophalangeal joint (MTPJ) instability is an important cause of forefoot pain and its causation multi-factorial. There is debate over an association with a relatively increased second metatarsal length and numerous measurement techniques. It is not known which, if any correlate with second MTPJ instability or their accuracy. We hypothesized that the method of measurement would affect the apparent relationship between second metatarsal length and second MTP instability. Materials and Methods: We analyzed standardized radiographs of 81 feet with second MTPJ instability using methods described by Coughlin, Maestro, and Hardy/Clapham. A control group of 11 feet were assessed from patients with a clinical diagnosis of Morton's neuroma (no instability) and successfully treated with a single steroid injection. The hallux valgus angle was also recorded. Patients mean age was 56 (range, 25 to 84) years, with female to male ratio of 4:1 and equal laterality. Results: Second metatarsal length measurements were not significantly different between grades of instability using any method, although Kruskal-Wallis analysis revealed a significant association between instability grades and increasing age (p < 0.05). Analysis of relative metatarsal lengths and second MTPJ instability revealed Coughlins method approached statistical significance (p < 0.055), but upon standardizing for age it became statistically significantly associated (r = 0.21, n83: p < 0.05). Spearman's rho correlation identified increasing hallux valgus was associated with decreased the second metatarsal length for Maestro and Hardy & Clapham methods (r = -0.29, n92: p < 0.005) and (r = -0.36, n86: p < 0.001), respectively. Conclusion: A positive correlation of second metatarsal length to second MTPJ instability existed using Coughlins method controlling for age. Maestro and Hardy & Clapham methods were possibly influenced by co-existing hallux valgus. The association of a clinical finding with a possible causative factor is likely influenced by its method of radiographic measurement.