TKA PATELLAR COMPLICATIONS

Overstuffing appears in the instability mnemonic and the biomechanics table ("measure pre-op thickness and reproduce it"), so it is worth defining the patellofemoral overstuffing mechanism it refers to.

• The patellar composite. After resurfacing, the patellar composite = residual patellar bone + the button; the aim is to reproduce the native patellar thickness (measured with a caliper before cutting). Native adult patellar thickness is roughly 22-25 mm, and about 12-15 mm of residual bone should be preserved to avoid fracture and osteonecrosis.
• Over-resection (residual bone under about 12 mm) weakens the patella and predisposes to fracture and AVN — compounded if the superior lateral genicular supply is also lost.
• Overstuffing (a composite thicker than native, or an oversized/proud anterior femoral flange) increases patellofemoral contact pressure and the extensor lever-arm demand, causing reduced flexion, anterior knee pain and higher shear that in turn drives maltracking, instability and component wear.
• Practical rule. Measure the patella before resection and resurface to reproduce native thickness; avoid an oversized or anteriorly-translated femoral component. Both errors are patellofemoral: too thin risks the bone, too thick risks the joint.

The exam warning tells you to differentiate patellar clunk from crepitus — the two sit on one spectrum of peripatellar fibrosynovial hyperplasia in posterior-stabilised (PS) knees, and the distinction matters for treatment.

• Same origin, different severity. Both arise from fibrous/synovial tissue at the quadriceps-patella junction interacting with the intercondylar box of a PS femoral component. Crepitus is a palpable or audible grating through the mid-flexion arc from softer peripatellar fibrosynovial hyperplasia; clunk is a discrete, often painful catch as a mature fibrous nodule snaps out of the box during terminal extension (around 30-45 degrees).
• Shared risk factors. PS designs, smaller/thinner patellar components, patella infera, a less accommodating trochlear box, and certain implant designs; the anterosuperior patella-quadriceps junction is the culprit zone.
• Treatment ladder. Crepitus is usually milder and often settles with observation and physiotherapy; clunk (a mechanical block) typically needs arthroscopic debridement of the nodule (the gold standard), which is also effective for refractory crepitus.
• The viva discriminator. Both preserve range of motion (unlike arthrofibrosis, which is stiffness). Crepitus = grating, often incidental; clunk = a painful, reproducible catch on extension. Naming both shows you understand the spectrum.

Strict adherence to range of motion limits is required for extensor repairs.