- How do I cope with lateral mal-alignment and subluxation?
80% of patients with patellofemoral arthritis have underlying mal-alignment and subluxation with lateral facet arthritis. The Avon patellofemoral arthroplasty is particularly successful in correcting mal-alignment without a distal tubercle transfer. It is, however, essential to follow the 15 steps outlined in the operative brochure, which explains the factors favourable to correct tracking and describes the procedures necessary for it to be effective.
Firstly, the design of the prosthesis itself based on the Kinemax Plus total knee replacement designed by Professor Peter Walker which is particularly favourable to satisfactory patella tracking. In full extension the patella is relatively unconstrained and, therefore, can assume its own position on the proximal trochlea as dictated by the anatomy and balance of the soft tissues. As the knee joint is flexed so the patella is captured by the trochlear groove and assumes greater stability and congruity.
Positioning of the femoral component with 3° to 6° of external rotation assists correct tracking, this necessarily moves the groove laterally by 2-3mm.Lateral translation of the femoral template by 1-2 mm will also assist.
The patella itself has a 3mm medial offset to the dome. Correct measurement of the thickness of the patella and a correct line of resection will ensure a symmetrical and balanced patellar implant.(see Design of the Avon Patella)
Once the prosthesis has been inserted it is important to ensure that there is correct balancing of the soft tissues. Important releases are performed on the lateral side of the joint, firstly the LATERAL PATELLOFEMORAL SYNOVIAL FOLDS low down where they reflect onto femur. Secondly, a release of the retinaculum where it inserts onto the lateral border of the patella, reflecting it off the lateral osteophyte, a so called SUB PERIOSTEAL PERI-PATELLAR RELEASE. The advantage of this is that it preserves the lateral geniculate vessels and does not broach the capsule of the knee joint.
Attention to the details of these finer points will usually allow normal tracking, which can be confirmed by the no-thumb test and the flip test. If there is slight lift-off at some point throughout the range of movement then the additional single suture test can be applied.
In very rare instances if there is still persistent mal-alignment then an Elmslie distal realignment osteotomy of the tubercle can be performed. This is secured with two 4.0mm fully threaded cancellous screws.
There are very rare cases of 100% subluxation or dislocation of the patella when it will be more satisfactory to approach the knee through a lateral incision and combine this with a distal tubercle transfer. In these instances the lateral retinaculum is necessarily left open, or reconstructed.
- Is it necessary to resurface the patella as I do not usually do so in performing a total knee replacement?
Isolated patellofemoral arthritis necessarily affects both parts of the patellofemoral joint. The patella is usually more severely damaged than the trochlea. Although the Avon femoral trochlea prothesis has a favourable shape to the normal patella, in most cases the patella has been distorted by the arthritic process and it is desirable to resurface the patella to create normal pain-free biomechanics of the articulation.
Careful attention to detail is required to balance the soft tissue envelope, firstly in performing the peri-patellar sub periosteal release from the lateral osteophyte and lateral border of the patella, and then dividing the lateral synovial fold.
It is also important to measure the patella before and after resection and after resurfacing to ensure that sufficient bone has been removed and there is no danger of over-stuffing the patellofemoral joint. Generally between 12mm and 15mm of residual patellar bone should be retained.
In rare circumstances either with a small patella or with significant erosion of patellar bone it will be necessary to resurface a patella with a significant defect of the lateral facet. Having performed the peri-patellar release the lateral osteophytes should be preserved to contain the defect and the cutting jig should be applied low on the medial side to ensure that there is a symmetrical resection of the patella. If there is an exceptionally thin patella, preferably never less than 10mm in thickness, then it may be necessary to shorten the lateral patella peg or use the extra small patella (to become available soon), which has shorter pegs.
In all circumstances it is desirable to resurface the patella otherwise there is a definite incidence of residual anterior knee pain, the reason for which the operation was performed. In cases where there is marked loss of patellar bone and the residual eggshell then it probably desirable to use a bone ingrowth reconstruction system or, if all else fails, a reshaping of the patella with a lateral facetectomy to create a smaller though congruous patella. (see case 4)
- What are the indications for patellofemoral arthroplasty?
It is important particularly when gaining early experience of this procedure that it is only performed when there is well-established symptomatic patellofemoral arthritis.
As further experience is developed and confidence in the procedure improves it will be possible to extend the indications to younger patients who have failed to respond to more conventional less invasive surgical procedures, such as chondrectomy and relignment procedures.
In patients under the age of fifty it is desirable to treat them with a less invasive biological solutions unless there is well-established arthritic change or significant trochlea dysplasia, which will prevent satisfactory articulation of the patellofemoral joint. See indications section.
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