Static and fatigue strength of a novel anatomically contoured implant compared to five current open-wedge high tibial osteotomy plates

Abstract

Background: The purpose of the present study was to compare the mechanical static and fatigue strength of the size 2 osteotomy plate "Activmotion" with the following five other common implants for the treatment of medial knee joint osteoarthritis: the TomoFix small stature, the TomoFix standard, the Contour Lock, the iBalance and the second generation PEEKPower.

Methods: Six fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy (HTO), according to standard techniques, using size 2 Activmotion osteotomy plates. All bone-implant constructs were subjected to static compression load to failure and load-controlled cyclic fatigue failure testing, according to a previously defined testing protocol. The mechanical stability was investigated by considering different criteria and parameters: maximum forces, the maximum number of loading cycles, stiffness, the permanent plastic deformation of the specimens during the cyclic fatigue tests, and the maximum displacement range in the hysteresis loops of the cyclic loading responses.

Results: In each test, all bone-implant constructs with the size 2 Activmotion plate failed with a fracture of the lateral cortex, like with the other five previously tested implants. For the static compression tests the failure occurred in each tested implant above the physiological loading of slow walking (> 2400 N). The load at failure for the Activmotion group was the highest (8200 N). In terms of maximum load and number of cycles performed prior to failure, the size 2 Activmotion plate showed higher results than all the other tested implants except the ContourLock plate. The iBalance implant offered the highest stiffness (3.1 kN/mm) for static loading on the lateral side, while the size 2 Activmotion showed the highest stiffness (4.8 kN/mm) in cyclic loading.

Conclusions: Overall, regarding all of the analysed strength parameters, the size 2 Activmotion plate provided equivalent or higher mechanical stability compared to the previously tested implant. Implants with a metaphyseal slope adapted to the tibia anatomy, and positioned more anteriorly on the proximal medial side of the tibia, should provide good mechanical stability.

Keywords: Activmotion- TomoFix; Biomechanics; ContourLock; Correction angle; Fatigue strength; High tibial osteotomy (HTO); Mechanical stiffness; Osteoarthritis; PEEKPower; Permanent deformation; Static strength; iBalance.

Fig. 1

Specimen and sensors’ locations: (a) Specimen before mounting to hydraulic press. b Specimen under test: The lateral and the medial sensor (LS and MS) register the relative lateral and medial vertical displacements from the tibial head, while VS measured its vertical displacement. The sensors DX, DY1 and DY2 register the horizontal displacements of the tibial head; along the transverse axis for the first and the sagittal axis for the latter

Fig. 2

Maximal displacement range within a hysteresis loop of an ideal spring-damper element: The hysteresis loop of an ideal spring-damper element is an inclined ellipsis

Fig. 3

Fracture of the lateral cortical bone in specimens: (a) Activmotion 1 and (b) Activmotion 2. The opposite cortex was the weak point of the specimens

Fig. 4

Static test results: (a) Activmotion 1: the rupture of the lateral cortex occurred without observable cracks formation. b Activmotion 2: Cracks formations preceded the final rupture of lateral cortex

Fig. 5

Fracture of the contralateral cortical bone during the fatigue testing: The specimens failed by fracture of the contralateral cortical bone, similar to the static tests

Fig. 6

Cracking of the contralateral cortex during the cyclical testing: Unlike the case of Activmotion 4 showed on the picture, the cracking was generally not observable

Fig. 7

Examples of hysteresis loops (Activmotion 4): Curves force versus lateral displacement. The maximal displacement range, which increase with the failure, is 0,07 mm

Fig. 8

Comparison of the deflection angle or valgus-malrotation of the tibial head before and after the failure for groups 1, 2, 3 and 6: The failure type 1 was observed in the case of the specimen iBalance 6 after the collapse of the opposite cortex. LS “n” means the failure occurred at load step “n”. The values of the first 3 groups are retrieved from our previous studies

Fig. 9

Average relative strength values: The TomoFix std. group has been taken as reference