What recurrent patellar instability actually is

If your kneecap has come out of place more than once — whether it fully dislocated and had to be pushed back, or simply gave way with a sickening lurch before you caught yourself — you are dealing with something different from a one-off injury. Recurrent patellar instability is the term for a kneecap that keeps failing to stay centred in its groove, and it has a distinct cause that repeated physiotherapy alone rarely corrects.

Episodes can range from a partial slip (subluxation), where the kneecap shifts but snaps back, to a full dislocation that is visibly obvious and intensely painful. Either way, patients commonly describe the same pattern: sudden sharp pain at the front or inner side of the knee, swelling within an hour or two, and — most disruptively — a persistent sense of apprehension. Stairs, sudden changes of direction, or simply a slight twist of the leg can all feel unreliable.

That lingering apprehension is not anxiety; it is the knee's honest signal that the mechanical system keeping the kneecap in place is compromised. When dislocation happens repeatedly, it points to an underlying structural reason — not simply a quad that needs more work.

Why the kneecap keeps dislocating — the structural reasons

Four structural factors — often overlapping — explain why one dislocation becomes a pattern.

Running from the inner edge of the kneecap to the inner femur, the medial patellofemoral ligament (MPFL) acts like a leash, limiting how far the kneecap can skid sideways. It is torn in virtually every lateral dislocation, and crucially, it does not reliably heal to a length and tension that restores normal restraint. Each episode therefore leaves the kneecap with less passive protection than it had before.

The shape of the trochlear groove — the channel at the base of the thigh bone — is equally important. A normal groove is V-shaped and draws the kneecap inward as the knee bends, like a ball settling into a rounded bowl. When that groove is flat or, in more severe cases, slightly convex, that bony capture is absent. This anatomical variant — trochlear dysplasia — is found in 68–85% of patients with recurrent instability, compared with just 3–6% of the general population; for many, it was present long before the first dislocation.

Patella alta adds a further disadvantage: a kneecap that sits too high does not reach the protective channel until later in the flexion arc, leaving it unsupported across a wider range of movement. Meanwhile, when the bony prominence where the patellar tendon attaches to the shin (the tibial tubercle) sits too far outward relative to the groove, every quadriceps contraction pulls the kneecap slightly laterally — a mechanical bias that compounds the other deficits. A tibial tubercle–trochlear groove (TT-TG) distance above 17 mm is generally considered elevated.

More than 80% of patients with recurrent instability have at least one of these features; many have two or three acting simultaneously. Identifying which factors are present, and to what degree, is precisely what guides the choice of treatment — the subject of the sections that follow.

Who is most likely to experience recurrent instability

Recurrent patellar instability is predominantly a condition of the young. Incidence peaks sharply in adolescents aged 14–18, with rates across children and adolescents reported at 23–43 per 100,000 per year — a window that coincides with rapid skeletal growth, increased sport participation, and the period in which underlying bony anatomy reaches its adult form. Young adults in pivoting or contact sport represent the other high-risk group, where activity level loads an already vulnerable anatomy repeatedly.

Females are affected at higher rates than males. This is not simply a matter of activity pattern: females tend to have measurably greater baseline ligamentous laxity and a wider pelvis-to-knee width ratio, which increases the lateral pull vector acting on the kneecap during every step and squat. Those differences amplify the effect of any trochlear or alignment abnormality that is already present. Where physical demand is extreme — military service data suggest roughly ten times the civilian instability incidence — the interaction between demanding load and underlying anatomy becomes even more apparent.

Joint hypermobility adds a separate layer of complexity. Generalised joint laxity is found in around 64% of patients presenting with patellar dislocation, compared with roughly 11% of controls. For patients with hypermobile Ehlers-Danlos syndrome in particular, standard MPFL reconstruction carries a failure rate of approximately 19% — meaningfully higher than the general surgical population — which is important context when weighing operative options.

A history of dislocation on the other side is also a recognised risk marker. Bilateral instability is not uncommon, and a contralateral episode should prompt a thorough assessment of underlying anatomy in both knees rather than treating the presenting side in isolation.

What conservative care can and cannot achieve

After a first dislocation — provided there is no loose body, large osteochondral fragment, or severely abnormal anatomy — supervised physiotherapy is the appropriate and evidence-supported starting point. Rehabilitation targets quadriceps strength (particularly the vastus medialis oblique, which applies a corrective medial pull on the kneecap), hip abductor control, and neuromuscular coordination, all of which reduce the lateral forces acting on the patella during movement and sport. A patella-stabilising brace can offer confidence and short-term symptom control, but it corrects nothing structurally — it is a management aid, not a remedy.

The honest limitation is anatomical. Physiotherapy cannot reshape a shallow trochlear groove or permanently re-tension a chronically lax MPFL. Where those structural deficits are significant, rehabilitation builds muscle support around an inherently vulnerable joint rather than resolving the underlying problem.

Published recurrence rates following conservative management alone range from 15% to 44%. That spread reflects how strongly anatomy drives individual risk: patients with trochlear dysplasia or patella alta sit at the upper end, while those with a first episode and minimal anatomical abnormality fare considerably better.

The clearest signals that conservative care has reached its limit are a second dislocation, persistent subluxation despite structured rehabilitation, cartilage damage confirmed on imaging, or an inability to return to normal activity. In a young athlete in pivoting sport, even a single recurrence in the presence of significant anatomical risk factors is often enough to prompt a surgical conversation.

Assessment and the decision to operate

Deciding whether surgery is appropriate begins well before any scan result. A consultant first takes a careful history — how many episodes have occurred, whether dislocations are becoming more frequent or triggered by progressively lower levels of activity, what sport demands the patient has, and whether persistent apprehension is preventing confident movement. Physical examination adds objective findings: the J-sign (the kneecap deviating sharply sideways during terminal knee extension) points to significant trochlear dysplasia, the apprehension test gauges soft-tissue restraint function, and a brief hypermobility screen identifies patients in whom ligamentous laxity may complicate any planned procedure.

Imaging then quantifies what examination has suggested. MRI establishes the condition of the MPFL, maps any cartilage damage, and identifies loose bodies or osteochondral fragments — each of which adds weight to the case for intervention. CT or MRI-based measurement provides two key numbers. The TT-TG distance — the offset between the tibial tubercle and the deepest point of the trochlear groove — reflects how far the quadriceps pull vector is directed laterally; values above 17–20 mm signal significant malalignment. The Caton-Deschamps index (CDI) quantifies patellar height: a value above 1.2 confirms patella alta, and figures approaching 1.4 attract stronger consideration of a distalising osteotomy alongside any soft-tissue work.

Trochlear morphology is graded using the Dejour classification from type A (a mildly shallow groove) through to type D (an absent groove with a bony crossing sign). Types B and D are the most clinically significant because they include a raised bony 'bump' that prevents the kneecap from seating correctly — a structural problem that quadriceps strength alone cannot resolve. These grades are the most likely to require correction of the groove itself, not just soft-tissue reconstruction.

No single measurement acts as a hard trigger for surgery. A consultant weighs the full picture — episode count, activity level, anatomy, cartilage health, and patient priorities — and it is that combination, rather than any one threshold, that guides the recommendation.

Surgical options and what to expect from stabilisation

Surgical stabilisation for recurrent patellar instability is not a single operation; procedures are combined from a 'menu à la carte', matched to each patient's anatomy rather than applied uniformly.

MPFL reconstruction is the anchor procedure when bony malalignment is absent or mild. A gracilis tendon autograft rebuilds the ligament from the superomedial patella to the medial femoral epicondyle. Across 521 knees at nearly eight years of follow-up, the re-dislocation rate is 5.75% — compared with 41% following simple repair at twelve-year follow-up (p = 0.019). Isolated lateral retinacular release is never appropriate as a standalone procedure for instability. In skeletally immature patients, physeal-sparing reconstruction techniques avoid growth plate injury; published series in patients averaging 14 years of age report a 5.1% recurrence rate with no major technique-related complications.

Tibial tubercle osteotomy is added when the TT-TG distance exceeds 17–20 mm or when patella alta is significant. Medialization corrects the lateral pull vector; distalisation lowers patellar height. In one adolescent series combining both with MPFL reconstruction, the Caton-Deschamps index fell from 1.34 to 1.04, Kujala scores rose from 47 to 94, and no further dislocations occurred at follow-up.

Trochleoplasty is reserved for Dejour type B or D dysplasia, where a raised bony trochlear bump physically obstructs patellar seating. Sulcus-deepening trochleoplasty combined with MPFL reconstruction achieved a re-dislocation rate of 1.6% and raised Banff Patellofemoral Instability Instrument 2.0 scores from 29.3 to 71.8 in a recent 63-knee series.

Long-term perspective. Stabilisation surgery is broadly effective — around 85–90% of patients remain dislocation-free — but seven-year follow-up after MPFL reconstruction identifies mild patellofemoral arthritis in approximately 20% of cases even after technically successful surgery. This reflects cartilage damage accumulated before stabilisation, not a consequence of the operation itself, and it is the clearest argument against allowing dislocations to continue unchecked. Where generalised joint hypermobility or hypermobile Ehlers-Danlos Syndrome is present, reconstruction failure rates may approach 19%, and surgical planning requires careful consideration of the broader ligamentous environment.

For most patients, a procedure combination matched accurately to their anatomy — undertaken before cumulative cartilage damage has narrowed future options — offers durable stability and a return to meaningful activity.

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