Bipartite Patella

The pathophysiology and controversies sections both flag that maltracking is "associated" with the painful bipartite and raise — without answering — whether tracking should be addressed. It is worth developing, because it reframes the symptomatic bipartite as partly a patellofemoral-overload problem, not purely a local fragment.

What the data actually show. According to PubMed (Atay 2023, 46 MRI-confirmed bipartite patellae, 17 symptomatic), the symptomatic group had significantly more frequent and more severe trochlear dysplasia — a higher trochlear sulcus angle (p=0.007) and a shallower trochlear depth (p=0.006) — and a more dysplastic patella morphology (Wiberg type III/IV), than the asymptomatic group. Crucially, there was no significant difference in the tibial-tubercle-to-trochlear-groove (TT-TG) distance (p=0.247) (DOI). So the association is with the trochlear and patellar shape, not with tibial-tubercle lateralisation.

The mechanism and the unresolved question. A shallow, dysplastic trochlea permits abnormal patellar tilt and tracking that concentrates load on the superolateral synchondrosis, tipping a previously-silent bipartite into a painful one. This explains why the painful bipartite clusters at the vastus-lateralis corner and why some are stubborn. It does not, however, translate into a realignment operation: because the abnormal parameter is the trochlea/patella shape rather than the TT-TG, tubercle-medialisation realignment is not indicated, and trochleoplasty is not standard for this benign condition. Current practice still treats the fragment (conservative, then excision or release), but recognising the dysplastic substrate justifies assessing and rehabilitating patellar tracking as part of conservative care. (The full patellofemoral-instability work-up — Dejour dysplasia grading, MPFL, TT-TG and trochleoplasty — belongs to the patellofemoral-instability topic.)

View larger

The universal scheme is the Saupe classification, which groups bipartite patellae purely by the location of the accessory fragment:

View larger

Imaging:

1. Plain radiographs (AP, lateral, skyline/tangential): the diagnosis is usually radiographic.
   • Site: superolateral corner (type III) most often.
   • Margins: smooth, rounded, corticated edges - the hallmark distinguishing it from a fracture.
   • Skyline view: best profiles the superolateral fragment.
   • Other knee: image it too - a bilateral, mirror-image ossicle strongly supports bipartite.
2. CT: confirms the separate corticated fragment and synchondrosis; useful when the radiograph is equivocal or to plan surgery.
3. MRI: the key test for deciding if a bipartite is the actual pain generator. Bone-marrow oedema straddling the synchondrosis (in the fragment and adjacent patella) indicates a symptomatic, painful synchondrosis; its absence suggests the bipartite is an innocent bystander. MRI also assesses trochlear dysplasia and tracking.
4. Bone scan / SPECT: increased uptake at the synchondrosis can localise the pain source when MRI is unavailable, though it is less specific.

View larger

Other entities to consider:

• Dorsal defect of the patella - a focal lucency/cyst within the bone, not a separate ossicle with a synchondrosis.
• Inferior-pole sleeve / avulsion fracture - the main confounder for the rare type I; here active extension may be lost and the history is acute.
• Patellar stress fracture - particularly in athletes; marrow oedema may overlap, but a fresh fracture line and donor site differ.
• Osgood-Schlatter / Sinding-Larsen-Johansson - apophyseal pain at the tibial tubercle or inferior pole in adolescents; can coexist.

The exam-warning and the differential both tell you not to confuse a bipartite patella with a dorsal defect of the patella, and a viva follow-up asks you to distinguish them — but the entity is never explained. It sits at the same superolateral corner, which is exactly why they are confused.

• What it is: a dorsal defect of the patella (DDP) is a well-defined, rounded, lucent defect in the subchondral bone of the superolateral patella, typically around 1 to 2 cm, filled with fibrous tissue. It is usually found incidentally in adolescents and young adults, is frequently bilateral, is almost always asymptomatic, and characteristically resolves spontaneously as the skeleton matures.
• The key difference from bipartite: a dorsal defect is a focal lucency within an intact, continuous patella — there is no separate ossicle and no synchondrosis. A bipartite is a discrete corticated fragment joined to the main bone by a fibrocartilaginous synchondrosis. On MRI the articular cartilage overlying a dorsal defect is intact, which both confirms the diagnosis and separates it from osteochondritis dissecans.
• The developmental link (why the topic says "do not merge them"): DDP and the superolateral (type III) bipartite are thought to share a developmental origin at the superolateral corner — the same region loaded by the vastus lateralis — and the two can coexist. They are nonetheless distinct lesions.

Why it matters: a dorsal defect is benign and self-limiting and needs no treatment — the danger is mistaking its lucency for osteochondritis dissecans, an infection or a tumour and over-investigating or biopsying it. Recognising the intact overlying cartilage on MRI and the typical superolateral, often-bilateral pattern settles it.

The guiding principle is simple: incidental bipartite needs no treatment; the rare painful bipartite is treated conservatively first, and surgically only if pain is refractory.

• Best operation is unsettled. Multiple systematic reviews agree that excision, lateral release, ORIF and arthroscopic variants all give good results, but the literature is entirely low-level (case series), so no technique is proven superior (McMahon et al 2016; Hines et al 2024).
• How long is "enough" conservative care? Thresholds for moving to surgery (often 3-6 months) are pragmatic rather than evidence-defined; earlier surgery within a relatively short window of refractory symptoms has been linked to better outcomes in younger patients in some reviews.
• Who actually gets symptomatic? The association with trochlear dysplasia and maltracking (Atay 2023) raises the question of whether tracking should be addressed alongside the fragment - currently not standard.
• Confirming the pain source. Because a bipartite can be an innocent bystander, MRI marrow oedema (or local uptake on bone scan) before committing to surgery is increasingly emphasised.

There are no dedicated national society guidelines for bipartite patella; practice is driven by systematic reviews and consensus. The picture below is consistent across regions.

Global epidemiology: prevalence is broadly quoted around 1-2% of the population worldwide, more common in males and frequently bilateral. There is no implant or arthroplasty registry relevance for this condition, as management is non-implant (conservative, fragment excision, or soft-tissue release).