The patella is a mobile fulcrum increasing the extensor mechanism's mechanical advantage at all positions of knee motion. The extensor mechanism is 50% weaker in a knee without the patella. The patella is subject to considerable forces, 5x body weight rising from a chair, 2x body weight going up stairs, and 20x body weight with jumping. Patellofemoral pressure increases with knee flexion and peaks around 120 degrees. The native patella articular surface is not symmetric, the medial facet is small and steeply angled, while the lateral facet is large with a shallow angle. The typical patellar implant is an all-polyethylene dome to prevent it from binding within the trochlear groove. The implant is placed along the medial border of the patella to improve tracking.
The first knee replacements only addressed the tibio-femoral articulation. These patients reported anterior knee pain in over 50% of cases . The first patellar resurfacing options appeared in the early 1970s, yet 6% experienced lateral patellar dislocation. Changes in TKA design and improved alignment techniques have significantly reduced this risk to < 1%. However, patella resurfacing still presents with its own unique risk of complication including fracture, mal-alignment and subluxation. Furthermore, while modern designs have reduced the incidence of anterior knee pain to about 10%, the number remains significant and thus the underlying etiology of anterior knee pain remains unclear.  .
1. Always resurface the Patella.
The majority of TKAs in the United States resurface the patella, however, the utility of this procedure remains controversial .
Resurfacing the patella decreases the risk of revision surgery for anterior knee pain. The Australian Registry Data reports a 1.4% higher rate of revision surgery in TKA without resurfacing, and other reports cite higher rates  ], up to 8% higher . The most common revision procedure is secondary resurfacing - indicating that the decision not to resurface is the number one reason leading to repeat surgery. However, this increased risk of revision surgery does not necessarily mean that resurfacing reduces the risk of anterior knee pain. The rate of revision only means that anterior knee pain in a TKA without patellar resurfacing is more likely to undergo surgery than anterior knee pain in a TKA with a patellar resurfacing (which makes sense from a practical standpoint as its easier for a surgeon to operate when there is something definite to do, ie resurface the patella, whereas, if the patella is already resurfaced, addressing anterior knee pain is often a more complicated procedure, and thus surgeons may encourage nonoperative treatments like physical therapy). Anterior knee pain in TKA with or without resurfacing is 10% on average and overall there is no difference between the two .
From a cost analysis standpoint, there is some evidence to support patellar resurfacing. While there may be controversy as to why the revision rates are higher for a non-resurfaced patella, the fact remains that revision rates are higher , and the cost associated with this difference more than compensates for any “savings” obtained by not implanting a patellar button. In summary, there is cost savings for resurfacing every patella .
It is best to primarily resurface the patella because there are unpredictable and generally poor results associated with secondary resurfacing for painful TKA when the patella was not resurfaced at the index procedure. Over 50% of patients report continued anterior knee pain and are dissatisfied with the procedure [11, 12]. Part of the problem is correctly diagnosing the patella as the cause of anterior knee pain. In patients with anterior knee pain and a bone scan identifying a “hot patella”, there were good results for secondary resurfacing. However, in patients with generalized knee pain and a “hot patella,” secondary resurfacing showed poor results . Secondary resurfacing in patients with knee pain and a “cold patella” similarly showed poor results .
From a biomechanical standpoint there is some evidence to support patellar resurfacing. The patella has the thickest cartilage in the body due to the high level of forces in the patellofemoral joint. And yet, a TKA further increases these forces 3x, exposing the patella to super-physiologic loads and possibly leading to acute symptoms or accelerated chondrolysis and later pain.
technical considerations. The patella is between 22-26 mm thick. After resurfacing, a minimum bony thickness of 12 mm is necessary to avoid fracture. The goal is to replicate the native patellar thickness, ie 22-26 mm. It is critical to obtain a flat patellar cut (commonly too much bone is resected from the medial side, increasing risk of fracture). Best results for patellar resurfacing include: maximizing size of patellar button without overhang (this decreases crepitus) , error on side of increased patellar thickness (concern for "overstuffing" is not demonstrated in literature), place component slightly superiorly to avoid patella baja, slightly medialize to improve tracking , inlay fixation is better than onlay.
2. NEVER RESURFACE THE PATELLA
There is not strong evidence to suggest that the un-resurfaced patella is a pain generator (despite the higher rates of revision surgery for un-resurfaced TKA).
A RCT examined the non-resurfaced TKA and found a low incidence of chondrolysis (7/46) that was not correlated with anterior knee pain . Another study examined bilateral TKA comparing one side resurfaced versus one side non-resurfaced, and failed to demonstrate significant differences .
A metaanalysis examined 13 RCT studies and found that rates of anterior knee pain were not statistically different between resurfaced and non-resurfaced TKA (10% overall)  and there was no difference in functional outcome scores. Among the 13 trials there were differences in outcomes. 5/13 found the resurfaced patella group had lower anterior knee pain, 1/13 found lower anterior knee pain with non-resurfaced patella, while 7/13 found no difference. Interestingly, when the 5 studies using TKA implants with better patellofemoral kinematics (“patellar friendly”designs) were isolated, non-resurfacing lead to better functional outcomes in 4/5. In studies of TKA designs without patellofemoral kinematic considerations “unfriendly designs”, the resurfaced patellar groups demonstrated better outcomes in 6/6 studies. Such findings suggest that TKA design has some effect on the outcome of resurfacing.
Anterior knee pain is probably not related to resurfacing but rather patellar tracking (based on implant positioning) and ligamentous balancing. Furthermore, advances in TKA design have developed more "patella-friendly" implants to improve patellofemoral kinematics and thus decrease anterior knee pain. Design changes include: deepening the trochlear groove and increasing conformity of patella to the trochlear groove, extending the anterior flange, adjusting the radius of curvature of femoral component to improve rollback and ligament isometry during flexion. These changes alone have had more of an impact on anterior knee pain than any studies suggesting resurfacing. The patellar offset and lateral patellar tilt are both decreased in a resurfaced patella, which appear to alter patellofemoral pressure and kinematics, as compared to a native knee, and may be related to anterior knee pain .
Resurfacing the patella is not a benign procedure. The complication rate for patellar resurfacing is between 3-7%.
3. Sometimes resurface
There are also those in the middle, who selectively resurface the patella in cases with obvious patella-femoral arthritis, patella-femoral knee pain preop, or inflammatory arthropathy, but preserve the native patella when there is no obvious pathology.
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