Abstract
Introduction
Opening wedge high tibial osteotomy is an attractive surgical option for physically active patients with early osteoarthritis and varus malalignment. Unfortunately use of this surgical technique is frequently accompanied by an unintended increase in the posterior tibial slope, resulting in anterior tibial translation, and consequent altered knee kinematics and cartilage loading(1).
To address this unintended consequence, it has been recommended that the relative opening of the anteromedial and posterolateral corners of the osteotomy are calculated pre-operatively using trigonometry (1). This calculation assumes that the saw-cut is made parallel to the native posterior slope; yet given the current reliance on 2D images and the ‘surgeon's eye’ to guide the saw-cut, this assumption is questionable.
The aim of this study was to explore how accurately the native posterior tibial slope is reproduced with a traditional freehand osteotomy saw-cut, and whether novel 3D printed patient-specific guides improve this accuracy.
Methods
26 fourth year medical students with no prior experience of performing an osteotomy were asked to perform two osteotomy saw-cuts in foam cortical shell tibiae; one freehand, and one with a 3D printed surgical guide (Embody, London) that was designed using a CT scan of the bone model. The students were instructed to aim for parallelity with a hinge pin which had been inserted (with the use of a highly conforming 3D printed guide) parallel to the posterior slope of the native joint.
For the purpose of analysis, the sawbones were consistently orientated along their mechanical and anatomical tibial axes using custom moulded supports. Digital photographs taken in the plane of the osteotomy were analysed with ImageJ software to calculate the angular difference in the sagittal plane between the hinge-pin and saw-cut. Statistical analysis was performed with SPSS v21 (Chicago, Illinois); a paired t-test was used to compare the freehand and patient-specific guide techniques. Statistical significance was set at a p-value <0.05.
Results
Using the traditional freehand technique, the mean difference in angle between the hinge pin and osteotomy saw-cut was 5.40 (SD 4.6), which contrasted with 1.40(SD 1) when the osteotomy was performed using a 3D printed guide [See Figure 1]. This difference was significant (p<0.001).
Discussion
This study highlights the large degree of error in the posterior slope of an osteotomy saw-cut when performed using a freehand technique, and which is likely to be a factor in the unintended change in tibial slope commonly observed in post-operative patients.
We found that a 3D printed patient-specific osteotomy guide significantly improved the accuracy and reduced the variability of this procedure. A follow-up multi-centre clinical trial is currently underway to ascertain whether these results are replicated in-vivo.
