Page 25 - ISAKOS 2018 Winter Newsletter

In a personal study, we found that the amount of external femoral rotation of an onlay prosthesis, as dictated by the axial orientation of anterior femoral cut, has an effect on the coronal orientation of the trochlear groove. Specifically, an increase in external rotation increased the valgus orientation of the groove, whereas an increase in internal rotation decreased the valgus orientation. The inlay type of implant can be used for the treatment of degenerative patellofemoral joints with normal or moderate trochlear dysplasia. The onlay type is more versatile and can be used for any knee, irrespective of the degree of trochlear dysplasia.
Technique
There are several important points to consider when performing a PFR. First, the surgeon must take care not to overstuff the patellofemoral compartment. This can be avoided by measuring the original thickness of the patella and resecting an amount of bone similar to the thickness of the selected patellar component. If there is severe patellar wear, the surgeon may need to resect less patellar bone than is being replaced. After resection, the thickness of the remaining patella should be at least 12 mm. Additionally, the anterior femoral cut should be flush with the anterior cortex of the femur, without notching.
The position of the trochlear component is also critical. With the onlay type of prosthesis, the orientation of the anterior femoral cut will influence both the prosthesis-articular cartilage transition and the coronal direction of the trochlea. There should be a smooth transition between the trochlear component and the remaining articular cartilage. If this is not possible, the prosthesis can be placed slightly lower than the surrounding articular cartilage, but never higher. Internal- external rotation of the anterior femoral cut will influence the coronal trochlear angle. More external rotation is necessary when a prosthesis with a 0° trochlear angle is used than is the case when a prosthesis with a built-in 7° valgus trochlear orientation is used. We prefer to use instrumentation that can predict the prosthesis-articular transition and the trochlear orientation before the anterior femoral cut is made. The size of the femoral component is important. Distal overhang might impinge on the anterior cruciate ligament (ACL), while a too- short proximal trochlea might result in the patella running on the articular cartilage and off the prosthesis in full extension. Medial and lateral overhang should be avoided.
If there is a choice between sizes, the smaller trochlear component should be selected. The patellar button should be placed medially, and lateral overhanging patellar bone should be removed as it can impinge on the femoral component (Fig. 2).
02
Finally, the surgeon must ensure that the patella tracks centrally (Fig. 3). Often, additional procedures are required, such as lateral facetectomy or lateral retinacular release. Occasionally, a tibial tubercle osteotomy is necessary. Dy et al., in a meta-analysis of PFRs, found that the most common additional procedure was lateral retinacular release, followed by tibial tubercle osteotomy.
03
Results
The intermediate-term results of PFR generally have been very good. In a recent systematic review of both first and second-generation designs, good to excellent knee function was reported in 87.3% of patients and the mean Knee Society Score was 87.5. Better outcomes have been seen in patients with trochlear dysplasia, whereas inferior outcomes have been associated with obesity and with low mental- health scores. These findings highlight the importance of patient selection when deciding on the optimal treatment for patients with patellofemoral degeneration.
There is a difference between the results associated with older (first-generation) designs and modern (second- generation) designs. Average annual revision rates of 2.14% have been reported. Revision rates have been reported to be higher with first-generation designs, mostly because of mechanical reasons. In most series, the primary cause for revision has been progression of tibiofemoral arthritis. Complications are more common with first-generation designs; however, there is no difference between designs with regard to the rates of persistent pain or progression of tibiofemoral osteoarthritis.
01 Different onlay designs.
02 PFR with smooth prosthesis-articular cartilage transition, no overhang,
and no sharp edges.
03 Preoperative and postoperative radiographs.
CURRENT CONCEPTS
ISAKOS NEWSLETTER 2018: VOLUME I 23