Abstract
Patellofemoral joint (PFJ) arthroplasty is traditionally performed using mechanical jigs to align the components, and it is hard to fine tune implant placement for the individual patient. These replacements have not had the same success rate as other forms of total or partial knee replacement surgery1.
Our team have developed a computer assisted planning tool that allows alignment of the implant based on measurements of the patient's anatomy from MRI data with the aim of improving the success of patellofemoral joint arthroplasty.
When planning a patellofemoral joint arthroplasty, one must start from the premise that the original joint is either damaged as a result of osteoarthritis, or is dysplastic in some way, deviating from a normal joint. The research aimed to plan PFJ arthroplasty using knowledge of the relationship between a normal PFJ (trochlear groove, trochlea axis and articular surfaces) and other aspects of the knee2, allowing the plan to be estimated from unaffected bone surfaces, within the constraints of the available trochlea.
In order to establish a patient specific trochlea model a method was developed to automatically compute an average shape of the distal femur from normal distal femur STL files (Fig.1). For that MRI scans of 50 normal knees from osteoarthritis initiative (OAI) study were used. Mimics and 3-matic software (Materialise) packages were used for segmentation and analysis of 3D models. Spheres were fitted to the medial and lateral flexion facets for both average knee model and patient knee model. The average knee was rescaled and registered in order to match flexion facet axis (FFA) distance and FFA midpoint of the patient (Fig.2). The difference between the patient surface and the average knee surface allow to plan the patella groove alteration.
The Patella cut is planned parallel to the plane fitted to the anterior surface of the patella. The patella width/thickness ratio (W/T=2) is used to predict the post reconstruction thickness3. The position of the patella component (and its orientation if a component with a median ridge is used) is also planned.
The plan is next fine-tuned to achieve satisfactory PFJ kinematics4 (Fig.3). This will be complemented by intraoperative PFJ tracking which assists with soft tissue releases. PFJ kinematics is evaluated in terms of patella shift, tilt and deviation from the previously described circular path of the centre of the patella.
The effect of preoperative planning on PFJ tracking and soft tissue releases is being examined. Additional study is needed to evaluate whether planning and intraoperative kinematic measurements improve the clinical outcome of PFJ arthroplasty.
