The purpose of this study was to evaluate the biomechanical strength of the use of a transtibial interference screw, for fixation of depression fractures of the lateral tibial plateau. Methods used were similar to previous published studies on tibial plateau fracture fixation [6, 7]. Our results demonstrate this novel fixation method provides comparable biomechanical results to traditional, parallel, subchondral screws, under normal physiological loads.
The technique of using a transtibial interference screw for fracture fixation of pure depression fractures of the tibial plateau was described in 2006 , but no clinical or biomechanical data has since been published to support its use. Previous studies have compared the number and size of subchondral screws used in traditional fixation, and the benefits of the use of bone graft, cement or trabecular metal to support elevated fragments [6–8]. The newer method of fixation, has perceived benefits. Arthroscopic reduction preserves the soft tissue envelope, negating the need for arthrotomy and meniscal detachment. Use of an interference screw allows simultaneous, precise fracture reduction under direct arthroscopic visualization. Reamed metaphyseal bone can be used as autograft. The technique eliminates the need for percutaneous buttress screw placement under fluoroscopic guidance, and avoids the soft tissue irritation that is sometimes associated with prominent percutaneous screws. The technique allows the possible introduction of bioabsorbable screw use. In this study only titanium screws were tested.
Our results demonstrated articular depression, following simulated post-operative loading, of on average 0.61 mm in fractures fixed with a single interference screw, and 0.76 mm in those fixed with parallel, subchondral screws. The goals of treatment of any intra-articular fracture include restoration of joint congruity, and prevention of secondary arthritis. Within the literature there is no definitive guidance on what is acceptable step off in articular cartilage following trauma. There are a number of studies, which have looked at this specific to the tibial plateau. Experimental models have shown acceptable depression of up to 3 mm before increased contact pressures are demonstrated in surrounding articular cartilage . Clinical studies, comparing depression fractures managed operatively and conservatively, have shown deterioration in outcome with articular depression greater than 3 mm . Others have shown no clinical difference despite up to 8 mm depression . Following cyclical loading, the average depression of stabilized fracture fragments in both groups of our study, was comfortably within the acceptable range.
A load of 20 kg/cm2 has been shown to correspond to the maximal loading of the knee during normal gait in a 70 kg man . Our study reflects the biomechanical strength of fracture constructs immediately after fixation and cyclical loading simulates early, full weight bearing in a patient of this size. No specimens failed during cyclical loading. Increased body weight or angular mal-alignment will increase the load passing through the lateral plateau. 15° of angular deformity increases maximal loading to 75 kg/cm2,  this would correspond to 1900 N in our study. Specimens ultimately failed at a mean of 1700 N in fractures fixed with a single interference screw. Protected weight bearing, or alternative fixation methods, should be considered in patients in whom load is expected to be out with normal physiological range.
We accept that our study has some limitations. Schatzker 3 fractures of the lateral tibial plateau, in which the tibial cortex remains intact with depression of the articular surface, are most commonly described in the elderly, with low level trauma . The porcine knees used in this study may not be representative of the poor quality, osteoporotic bone seen in this group. Specimens in this study were disarticulated and stripped of soft tissue prior to simulation of the depression fracture. No cortical fracture could be seen on close inspection of the tibia but the possibility of microfracture could not be excluded without X-ray or CT. Using a cadaveric model has inherent drawbacks, tissue properties may have been affected by the preservation process. Mechanical testing has shown that fracture stress values in canine and porcine bone, most closely resemble those of human bone . In both cadaveric and animal models there is often a wide variation in the mechanical properties of samples. In order to test mechanical construct strength, the ideal samples would be of identical bone quality. In this study, specimens were randomly assigned to fixation method in attempt to reduce the impact of variation in bone quality. The mechanism of fracture failure did not represent what we would expect in vivo. In this study we did not load the entirety of the tibial plateau, reducing the accuracy of simulated physiological loading. Loading over a larger area may prevent propagation of a cortical split from the original fracture depression. No consideration has been made to the effects of fracture healing.