Primary fixation strength of patellar fixation methods for medial patellofemoral ligament reconstruction in a porcine model

AIMS AND OBJECTIVES: The reconstruction of the medial patellofemoral ligament (MPFL) with autologous tendon grafts is an established method for stabilisation of the patella in case of recurrent instability. There are several operative techniques described in literature that differ i.a. in their patellar fixation methods. Techniques with transpatellar drilling show higher complication rates, the use of anchors or interference screws lead to additional costs. Soft tissue fixation methods have also been described but there is a lack of biomechanical data. The aim of this study is to test biomechanical properties of implantfree patellar fixation methods. MATERIALS AND METHODS: We used a porcine patella model for biomechanical testing. From 60 domestic pigs patellae and extensor tendons were harvested and randomized to 6 test groups. One test group (n = 10) was used to measure primary stability of the native tissue. In the other five groups different patellar fixation methods were examined (n = 10). In two groups the free tendon graft was interlaced in a u-typed manner and fixed with absorbable suture USP 0 (group 1: fixation with four sutures, group 2: fixation with three parallel sutures). In two groups the free tendon graft was fixed with a transosseous suture USP 0 (group 3) and with non-absorbable suture USP 3 (group 4). In group 5 fixation was performed with a v-shaped osseous tunnel with 4,5 mm diameter. The groups were compared in relation to maximum fixation strength (Fmax), yield load, stiffness and elongation after 1000 cycles with loading force of 50 N. Testing was performed with a material testing machine (Zwick/Roell Z010, Ulm, Deutschland). For statistical analysis IBM SPSS Version 25 was applied. Differences between the groups were calculated with Kruskal-Wallis test. Significance level was < 0,05. RESULTS: Implantfree fixation methods in the porcine model show particulately similar fixation strength compared to the native tissue (p < 0,05). The mean Fmax of the native tissue was measured with 252,1 ± 129,3 N. The highest Fmax were observed in group 1 and group 2 (321,8 ± 53,5 N and 242,0 ± 57,4 N). The other groups showed lower Fmax (p < 0.05). Yield load was 240,8 ± 129,7 N for the native tissue. The highest yield load was observed in group 1 (285,6 ± 72,2 N). The yield load in all other groups was lower. Stiffness of the native tissue was 45,42 ± 10,8 N/mm² and comparable to group 1 (45,08 ± 13,3 N/mm²) and group 2 (47,47 ± 10,2 N/mm²). Group 5 achieved the highest stiffness (61,42 ± 10,5 N/mm²). The lowest elongation was observed in group 5 (1,36 ± 0,4 mm). Group 1 showed elongation of 3,5 ± 0,7 mm, group 2 3 ± 0,5 mm, group 3 5,28 ± 1 mm and group 4 3,7 ± 0,8 mm. CONCLUSION: The examined implantfree patellar fixation methods differ significantly in their biomechanical properties (p < 0,05). Adequate Fmax compared to the native tissue could be observed. Soft tissue fixation of free tendon grafts for medial patellofemoral ligament reconstruction are reasonable alternatives for patellar fixation methods.