Impact of First Metatarsal Hyperpronation on First Ray Alignment: A Study in Cadavers.

There is increased evidence of first metatarsal hyperpronation in patients with hallux valgus, but its impact on the stability of the first metatarsophalangeal and metatarsosesamoid joints is unknown. A previous biomechanical study showed that an increase in hallucal pronation might lead to medial soft tissue failure of the first metatarsophalangeal joint. Conversely, dynamic studies on hallux valgus have shown that the first tarsometatarsal joint moves in supination during weightbearing, and supination was associated with an increase in the intermetatarsal angle (IMA) and hallux valgus angle (HVA).(1) Does an increase in first metatarsal pronation cause an increase in hallucal pronation? (2) Can an intrinsic increase in first metatarsal pronation lead to first ray supination during weightbearing? (3) Can a combination of intrinsic first metatarsal hyperpronation and first metatarsophalangeal medial soft tissue failure increase supination of the first ray during weightbearing? (4) Is first ray supination during weightbearing associated with an increase in the IMA and HVA?Twelve transtibial, nonpaired cadaver specimens without deformities were used. Each specimen underwent six weightbearing CT scans under different conditions. The first three CT examinations were performed without any osteotomy of the first metatarsal. The first was a simulated nonweightbearing condition. The second was a simulated weightbearing condition. The third was a simulated weightbearing condition with medial soft tissue release. Subsequentially, a 30° pronation osteotomy of the first metatarsal was performed, and the same sequence of weightbearing CT images was obtained. On each weightbearing CT image, the HVA, IMA, sesamoid rotation angle, metatarsal pronation angle (MPA), metatarsosesamoid rotation angle, and hallucal pronation (HP) were measured. Motions were calculated based on the differential values of these angular measurements produced by the six different conditions (weightbearing, medial soft tissue release, 30° pronation osteotomy, and combinations of these conditions). We compared means using a t-test for normally distributed variables and the Mann-Whitney U test for nonnormally distributed variables. Correlations were assessed with Pearson product-moment correlation coefficients.We found that 30° pronation osteotomy of the first metatarsal increased the MPA and HP by 28° ± 4° and 26° ± 6°, respectively, in the nonweightbearing condition. No differences between the increase in MPA and the increase in HP were noted (mean difference 2° [95% CI -1° to 5°]; p = 0.20). Therefore, an increase in first metatarsal pronation caused an increase in hallucal pronation. When a 30° pronation osteotomy of the first metatarsal was performed, the first ray motion during weightbearing went from pronation to supination (4° ± 2° in pronation without osteotomy versus 4° ± 2° in supination after the osteotomy, mean difference 8° [95% CI 6° to 9°]; p < 0.001). Therefore, an intrinsic increase in pronation of the first metatarsal led to a first ray supination motion during weightbearing. When a first metatarsophalangeal medial soft tissue release was performed in addition to the 30° osteotomy of the first metatarsal, the supination motion of the first ray increased (4° ± 2° without medial soft tissue release versus 11° ± 7° after the release, mean difference 8° [95% CI 3° to 12°]; p = 0.003). Therefore, a combination of intrinsic first metatarsal hyperpronation and first metatarsophalangeal medial soft tissue failure increased supination of the first ray during weightbearing. Regarding static angular measurements, the HVA and IMA were not correlated with the MPA (ρ = 0.20; p = 0.09 and ρ = 0.22; p = 0.07, respectively). Regarding motions, as the HVA and IMA increased from nonweightbearing to weightbearing the pronation decreased, with strong correlations (ρ = -0.82; p < 0.001 and ρ = -0.77; p < 0.001, respectively). Therefore, a first ray supination during weightbearing was associated with an increase in the HVA and IMA.The combination of first metatarsal intrinsic hyperpronation and first metatarsophalangeal medial soft tissue failure led to a hallux valgus deformity in this cadaveric study. The static measurement of first metatarsal head pronation relative to the ground (MPA) did not reflect the real intrinsic pronation of the first ray, and foot and ankle specialists should be careful when interpreting these measurements. Hallux valgus is a dynamic condition, and the deformity could be more correlated with motions during weightbearing than with plain static measurements.First ray supination compensating for first metatarsal intrinsic hyperpronation might be an important factor in the hallux valgus pathogenesis. Further in vivo studies involving nonweightbearing and weightbearing comparative assessments of hallux valgus and controls should be performed to confirm this pathomechanism.