Peak Contact Stress of TMT-1 Joint after Sequential Correction of Hallux Valgus Using a Proximal Opening Wedge Metatarsal Osteotomy (PMO) and Distal Soft Tissue Procedure

CATEGORY: Bunion; Midfoot/Forefoot INTRODUCTION/PURPOSE: The proximal opening wedge metatarsal osteotomy (PMO) of the first metatarsal is a common procedure for the surgical treatment of moderate to severe hallux valgus. Although with a long track record of success, limited work has been dedicated to this procedures effect on the first tarsometatarsal (TMT-1) joint. The purpose of this study is to evaluate the changes in TMT-1 joint contact stress in the diseased and corrected state following an opening wedge osteotomy of the proximal metatarsal. In addition, the effect of a distal soft tissue release (DSTR) was evaluated as it related to both radiographic correction and TMT-1 joint contact stress. METHODS: Seven fresh-frozen cadaveric below knee specimens (mean age: 69yrs) with hallux valgus deformities (mean HVA: 31.7+-12.0degs) were obtained for the study. The specimen was loaded up to 400N on an MTS servo hydraulic load frame with the tibia at 90-degree to the neutrally position foot. Joint contact characteristics at TMT-1 joint were measured with a Tekscan pressure sensor (Model6900, 1100psi). A standard proximal metatarsal osteotomy was performed. Various sized metal wedges (3, 5, 7 mm) with locking plates and screws were inserted in the osteotomy for correction. Following initial tests, a complete distal soft tissue release (DSTR) was performed and the specimens were retested. Additionally, dorsoplantar weight bearing (400N) radiograph was obtained for each condition to measure intermetatarsal (IMA) and hallux valgus (HVA) angles. The contact force, area, and peak contact stress were compared among groups using ANOVA and post-hoc multiple comparisons over the untreated (Dunnett test, p<0.05). RESULTS: The mean HVA decreased with wedge size and DSTR, reached to significant level with 7mm+DSTR (24.1 degs). The mean contact force was 39.7+-32.6 N for untreated specimens. This increased sequentially with opening wedge size and reached statistical significance 7mm opening-wedge (119.6+-53.8 N, p=0.03) and 7mm-wedge+DSTR (116.7+-58.3 N, p=0.04). The peak contact stress followed a similar trend (Figure 1). The mean peak contact stress was 2.3+-1.5 MPa for the untreated specimens and increased incrementally with wedge size to 5.3+-2.6 MPa for 7mm-wedge only (p=0.03) and 5.2+-2.1 MPa for 7mm- wedge+DSTR (p=0.04). Contact area increased with corrections, but none reached significance. CONCLUSION: The results from this study demonstrate that with sequentially increasing opening wedge size, loading properties through the TMT-1 joint increase. Prior work has demonstrated that joint stresses of over 4.7 MPa can be chondrotoxic, a value which was surpassed with our peak contact stress with the 7mm wedge. This has significant implications for the long-term health of the TMT-1 joint following PMO, potentially predisposing patients to arthritic joint changes. The optimal degree of correction with PMO to limit chondrotoxicity is not known at this time, and is a direction for future work.