Graft Extrusion Related to the Position of Allograft in Lateral Meniscal Allograft Transplantation: Biomechanical Comparison Between Parapatellar and Transpatellar Approaches Using Finite Element Analysis

Purpose: To compare the relation of extrusion of the graft with the position of the allograft between the parapatellar and transpatellar approaches and to show the primary importance of an anatomically correct position by comparing the chondroprotective effects after lateral meniscal allograft transplantation (MAT) with those of normal healthy knees. Methods: Geometrical data from patients who underwent magnetic resonance imaging evaluation after lateral MAT were used as baseline input data for 3-dimensional and finite element analysis. The inclusion criteria were patients with symptomatic knees that had undergone meniscectomy who underwent lateral MAT with a minimum follow-up of 2 years. Patients with generalized arthritis, lower limb malalignment with greater than 5 degrees valgus or varus, or uncorrected joint instability caused by ligament structure deficiency were excluded from this study. Patients were divided into the parapatellar group (25 patients) and transpatellar group (20 patients) according to surgical approach. Results: The mean width of the extruded meniscus was 4.32 +/- 0.58 mm in the parapatellar group and 3.00 +/- 0.61 mm in the transpatellar group (P < .0001). The mean relative percentage of extrusion was 42.48% +/- 7.82% in the parapatellar group and 28.21% +/- 4.49% in the transpatellar group (P < .0001). The mean angle between the bony bridge and the center of the tibial plateau was significantly greater in the parapatellar group (16.69 degrees +/- 2.68 degrees) than in the transpatellar group (5.29 degrees +/- 1.55 degrees, P < .0001). The mean distance from the entry point of the bony bridge to the center of the tibial plateau was also greater in the parapatellar group (16.68 +/- 2.56 mm) than in the transpatellar group (10.81 +/- 1.37 mm, P < .0001). The distance from the entry point of the bony bridge to the center of the tibial plateau significantly influenced the obliquity of the bony bridge in the parapatellar group (P = .002). On finite element analysis, the transpatellar approach was more similar to the intact knee model in terms of the contact area and stress of the lateral meniscus and medial meniscus as well as the maximum compressive and maximum shear stresses. Compared with the parapatellar approach, the transpatellar approach had lower maximum contact stress on the menisci and lower maximum compressive stress and maximum shear stress on the femoral and tibial articular surfaces. Conclusions: The transpatellar approach led to a more anatomically correct positioning of the grafted meniscus with less meniscal extrusion than did the parapatellar approach in lateral MAT. Furthermore, the transpatellar model had lower maximum contact stress on the menisci than did the parapatellar model, and it also had lower maximum compressive stress and maximum shear stress on the femoral and tibial articular surfaces.