Using CT generated 3D models to measure the bony geometry and mechanical axes of the lower limb before and after performing a computer navigated opening wedge high tibial osteotomy

Introduction: Obtaining exact information on the bony geometry and mechanical axes of a patient is crucial in pre-, intra-, and post-operative planning for orthopaedic procedures of the lower limb. Procedures such as high tibial osteotomies (HTO) alter the mechanical axis such that loads are moved away from an arthritic joint compartment 1,2. Conventional calculations have been performed using 2D radiography or intra-operative fluoroscopy with diathermy, but have limitations 3. Computer-assisted navigation requires the rigid fixation of optical markers to bone in order to provide the surgeon continuous visualization of mechanical axes during surgery. Unfortunately, landmarks can also be registered percutaneous-ly which could affect the accuracy of these axes. For example, the intra-operative detection of the transepicondylar axis has shown poor repeatability 4. In this study, pre-operative computed tomography (CT) data was converted into 3D computer bone models such that landmarks could be placed directly onto bone, thereby improving the accuracy of anatomic measurements 5. The objective was to develop a novel process for identifying specific bony landmarks of 3D CT models of cadaveric lower limbs, and use them to measure their geometry and calculate their associated mechanical axes. An opening wedge HTO was also performed to examine changes in measurements. With the implantation of a 12.5mm tapered Puddu plate (Arthrex, FL) we would expect to see a 12.5° increase in valgus angle of the knee joint 6. In addition, the orientation of the tibial plateau and leg length after HTO is not anticipated to change. Materials and Methods: The lower extremities of 8 thawed cadavers (7 male, 1 female) were obtained, but 3 limbs were lost due to inadequate bone quality for plating. Therefore, a total of 13 limbs were analyzed. The average age was 68yrs (range 35-92). The feet were held in foam mold at approximately 15° hip flexion with full extension of the knee. Each limb was scanned before and after each HTO procedure. The osteotomy plane was established using VectorVision1.0 (BrainLAB, Munich/Germany) navigation system, and the plate was inserted and screwed medially into place. The cadavers were scanned using a GE LightSpeedVCT with scan spacing of 2.5mm at regions of interest: the hip, knee, and ankle. This CT data was converted into 3D computer models using Mimics10.11 (Materialize, Leuven/Belgium), and landmark points were placed directly onto the surface (Figure 1). Spherical landmark center points of the femoral head and postero-distal aspects of femoral condyles were calculated using …