Patellofemoral arthroplasties (PFA) are orthopedic procedures where the patella and femoral trochlear articular surface are replaced by prostheses 1. This differs from unicompartmental knee arthroplasties, which replace the medial or lateral articular surface of the knee 1. Globally, PFAs make up ~1% (range 0.6-1.1%) of primary knee arthroplasties 1,2.
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History
The first PFA was proposed in 1955 by McKeever to avoid patellectomy in patients with isolated patellofemoral arthritis 3. There have been mainly two generations designs since the the first PFA. The latest designs aim to improve the patella tracking and reduce the revision rates.
Indications
Appropriate selection is important to reduce revision rates in PFA. Indications include 3:
isolated patellofemoral arthritis
intact knee extensor mechanism
age >40 years
BMI <35
Comparison
Compared to TKA, patellofemoral arthroplasties have 4:
shorter surgical times, reduced blood loss, and faster recovery times
ligament preservation
improved function
more cost-effective
Contraindications
Contraindications include 3:
patellar malalignment
uncorrected coronal plane deformity (valgus >8° and varus >5°)
sagittal plane deformity
knee instability
Relative contraindications include 3:
higher BMI
Prosthesis design
The first-generation protheses were an inlay design that replaced the trochlear cartilage only and left the subchondral bone intact 3. These initial designs had a high revision rate. Modern inlay designs have since been introduced which aim to recreate the complex anatomy and kinematics of the patellofemoral joint 5.
Second-generation prostheses have an onlay design, that uses an anterior femoral cut to replace the whole trochlear with a prothesis 3.
The two components are made of different materials:
patella component: cemented polyethylene button
femoral component: metallic anatomic conforming femoral component
Further design progression includes improving the anatomical conformity of the femoral component, and improving the trochlear flange and the patella button to improve the patellar tracking ref.
The level of constraint can be increased at either the femoral or patellar side:
trochlea: constraint can be increased by deepening the trochlear groove, changing the radius of curvature or changing lateral trochlear ridge 3
patella: constraint can be increased by having a more conforming patella button to the implanted trochlea; the trade-off is that it is less forgiving and more prone to malpositioning 6
Radiographic features
Plain Radiograph
AP projection
This view may be helpful in reviewing the rotation of the femoral component and direction of the trochlear groove.
Skyline projection (horizon)
Pre-operatively, this view assesses patellar tilt, displacement and subluxation.
Post-operatively, this view can be used to ensure the operative plan was carried out and re-assess the parameters above.
Lateral projection
Pre-operatively, this view assesses patellar height.
The pre- and post-operative views can be compared to look for overstuffing of the post-operative patella.
CT/MRI
Pre-operatively, a CT scan or an MRI may be used to help assess the patient’s Q-angle and their lower limb rotational profile. This may include assessing the tibial tubercle-trochlear groove (TT-TG) distance, rotational malalignment and patient bone stock 3.
CT
Metal artifact reduction reconstruction can assist in answering the following clinical questions:
implant assessment, e.g. component rotation or overhang, polyethylene wear
assessment for fractures or microfractures
assessing for osteolysis, i.e. aseptic loosening, periprosthetic infection
MRI
Pre-operatively, an MRI can assess for trochlear dysplasia and the quality of intra-articular ligamentous and fibrocartilage structures.
Post-operatively, an MRI may be used to assess for a cause for post operative pain such as early tibiofemoral arthritis. The amount of information achievable may be limited due to metal artefact.
Nuclear medicine
Bone scintigraphy may be useful in the case of a painful PFA to look for radiotracer uptake and assess for potential aseptic loosening.
Complications
Potential complications of PFA include 3:
aseptic loosening
hardware failure, e.g. polyethylene wear
patellar maltracking or instability
patella overstuffing
lateral catching
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