TKA PATELLAR COMPLICATIONS

Patellar complications after total knee arthroplasty are fracture, instability, clunk syndrome and aseptic loosening.

How common. In the 1980s and early 1990s patellofemoral problems were widely reported as accounting for a large share of TKA reoperations. The "up to 50%" figure still quoted is from that era and is not sourced to a single study, so quote it as history. Modern trochlear geometry, better rotational instrumentation and selective resurfacing have changed the picture, and contemporary revision series place aseptic loosening, infection, polyethylene wear and instability ahead of patellofemoral causes.

Fracture. Patellar fracture itself is uncommon, at 0.68% of 12,464 arthroplasties in the largest denominator-based series (Ortiguera and Berry, carded below). The risk factors fall into three groups:

• Vascular: lateral release (AVN)
• Technical: over-resection (residual bone less than 12 mm) and excessive thermal necrosis
• Patient: osteoporosis, high activity and male sex

The blood supply. The patella is supplied primarily by the genicular anastomosis, in which the superior lateral genicular artery and the inferior lateral genicular artery are the most critical vessels. The superior lateral genicular artery runs in the substance of the lateral retinaculum, and a wide lateral retinacular release sacrifices it. That significantly increases the risk of patellar AVN and subsequent fragmentation or fracture, so leave a cuff of tissue if possible.

Technical errors. Each of these has an effect on the patella and a surgical counter:

The patellar composite. After resurfacing, the composite is the residual patellar bone plus the button, and the aim is to reproduce the native patellar thickness, measured with a calliper 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.

Too thin. Over-resection, leaving residual bone under about 12 mm, weakens the patella and predisposes to fracture and AVN. The risk is compounded if the superior lateral genicular supply is also lost.

Too thick. Overstuffing is a composite thicker than native, or an oversized or proud anterior femoral flange. It increases patellofemoral contact pressure and the extensor lever-arm demand, causing reduced flexion, anterior knee pain and higher shear, which in turn drive maltracking, instability and component wear.

The femoral side. Oversizing or a proud anterior contour can overstuff the patellofemoral compartment even when the patellar composite thickness is restored. Anterior stuffing can be quantified by comparing the anterior chamfer bone resection with the corresponding implant thickness: a component thicker than the bone removed increases anterior offset and patellofemoral pressure.

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The rule. Measure the patella before resection and resurface to reproduce native thickness, and avoid an oversized or anteriorly translated femoral component. Both errors are patellofemoral: too thin risks the bone, too thick risks the joint.

One spectrum. Patellar crepitus and patellar clunk sit on one spectrum of peripatellar fibrosynovial hyperplasia in posterior-stabilised (PS) knees. Both arise from fibrous and synovial tissue at the quadriceps-patella junction interacting with the intercondylar box of the 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 at the superior pole snaps out of the box during terminal extension, at around 30-45 degrees.

Risk factors. The two share them: PS designs, smaller or thinner patellar components, patella infera, a less accommodating trochlear box and certain implant designs. The culprit zone is the anterosuperior patella-quadriceps junction.

Telling them apart. Both preserve range of motion, which separates them from arthrofibrosis, a stiff knee. Crepitus is a grating and often incidental; clunk is a painful, reproducible catch on extension and must also be told apart from instability. The distinction matters for treatment, and naming both shows you understand the spectrum.

History. Each complication has its own story:

• Instability: "the knee gives way", "the patella jumps out"
• Clunk: a painful catch or clunk as the knee extends from flexion
• Fracture: acute pain and inability to straight leg raise
• Pain: anterior knee pain, worse on stairs and rising

Examination. Look at the Q-angle and for valgus alignment, and palpate for tenderness and the fibrous nodule of a clunk. Assess patellar tracking for a J-sign, and the implant for gross loosening or instability.

The straight leg raise. The test that must be done is an active straight leg raise. Inability implies a Type III or IV fracture or a tendon rupture requiring surgery, and an absent raise or an extensor lag calls for urgent surgical planning.

Radiographs. AP and lateral films show the fracture, implant position and bone stock. The skyline (Merchant) view is critical for patellar tilt, subluxation and the fracture profile, and it is essential when AP and lateral films understate patellofemoral failure.

What the skyline finds. Severe patellar resorption, a pattern consistent with patellar osteonecrosis, is one. Lateral-facet impingement is another: contact produced by a medialised patellar button and a prominent prosthetic trochlear margin, or facet wear in an unresurfaced patella, is a mechanical pain source that AP films can miss.

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CT rotational profile. CT is indicated for instability or anterior knee pain, and measures the rotational alignment of the femoral and tibial components. Femoral rotation is read from the posterior condylar axis against the transepicondylar axis, tibial rotation from the geometric centre against the tubercle. A confirmed dislocation needs skyline imaging and CT assessment of component rotation before soft-tissue treatment or revision is chosen.

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Bone scan. For pain with normal radiographs and CT, a bone scan looks for occult loosening. Increased uptake in a "hot" patella may suggest stress fracture or loosening.

After extensor repair. Strict adherence to range-of-motion limits is required. Serial review must also assess fixation, graft protection and recurrent patella alta.

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Fracture. Surgical repair fails in 20-30% of cases. Nonunion is common, at 30-50%, but often painless if Type I, and the best outcome is a Type I fracture treated non-operatively, with 96% success.

Clunk. Arthroscopic debridement has a success rate greater than 95%, and recurrence is rare if enough of the nodule is resected.

Instability. Success depends on identifying the cause. Revising malrotated components succeeds in 80-90%.

Extensor rupture. The prognosis is poor, and primary repair has a high failure rate. In a TKA, chronic tissue compromise makes reconstruction more reliable than isolated primary repair, and allograft or mesh reconstruction is often required. Arthrodesis may be the end stage.

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Related pages: TKA Extensor Mechanism Failure is the endpoint the worst of these complications reaches, and the two pages share a governing principle — extensor mechanism integrity decides management; Patella Fractures for the native-knee injury and its fixation, and Patellar Height Abnormalities for the alta and baja that both cause and result from these problems; TKA Alignment Philosophies and TKA Alignment Options carry the rotational landmarks — transepicondylar axis, Whiteside's line, medial third of the tubercle — whose misuse produces the combined internal rotation the Berger card measures; TKA Complications: Instability because patellar maltracking and tibiofemoral instability share malrotation as a cause and are frequently confused; TKA Aseptic Loosening for the patellar component loosening that defines a Goldberg Type III / Ortiguera Type III fracture; Revision TKA for the component revision that malrotation ultimately requires, since no soft-tissue procedure corrects a malrotated component; Knee Arthrofibrosis and TKA Stiffness for the fibrosynovial proliferation at the root of the clunk-crepitus spectrum; Periprosthetic Joint Infection because a painful patellofemoral joint after arthroplasty is infection until excluded; Patellar Instability Pediatric for the native tracking mechanics that the arthroplasty is trying to reproduce; and Polyethylene UHMWPE and XLPE for the bearing material behind patellar component wear.