Patellar Height Abnormalities (Patella Alta and Baja)

Patellar height describes where the patella sits relative to the femoral trochlea and tibial plateau on the lateral knee radiograph. A patella that sits too high is patella alta; one that sits too low is patella baja. Both alter the biomechanics of the patellofemoral joint, and each has specific clinical consequences.

Alta is the more common abnormality and a well-established risk factor for recurrent patellar instability. Baja is less common but is an important iatrogenic problem after knee surgery, particularly total knee arthroplasty and tibial tubercle osteotomy.

Why it is examined. Patellar height is a measurable, correctable risk factor, and it should be assessed in every patient with patellar instability and every post-TKA patient with limited flexion. Three themes recur: how to measure it accurately (which view, which ratio, which thresholds), what alta and baja mean clinically (instability versus stiffness), and when and how to correct it surgically.

It surfaces across the exam. In the radiology viva you may be handed a lateral knee X-ray to measure and interpret; in the clinical viva alta is an integral part of the instability work-up; in the basic science viva, how patellar height affects trochlear engagement and contact pressures links anatomy to clinical decision-making.

The patella's role. A sesamoid bone within the quadriceps extensor mechanism, the patella increases the moment arm of the quadriceps by up to 30 percent and distributes compressive forces across the femoral trochlea. In flexion it tracks within the trochlear groove, engaging it at approximately 20 to 30 degrees and deepening its congruence as flexion increases.

Its restraints. Stability depends on static constraints (trochlear geometry, the MPFL and medial patellofemoral complex, bony architecture) and dynamic constraints (vastus medialis oblique, quadriceps balance). Patellar height directly affects the timing of trochlear engagement.

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Patella alta. A high patella, on a long patellar tendon, enters the trochlear groove later in the flexion arc, typically beyond 30 to 40 degrees. Through those first 30 to 40 degrees it is perched above the groove and relies almost entirely on soft-tissue restraints, principally the MPFL, for stability. It spends more time in the unstable early-flexion zone, the range where most daily activities occur, and the risk of lateral patellar dislocation is significantly increased, especially in early flexion or with a valgus and external rotation force.

Alta also increases patellofemoral contact pressures in mid-flexion, because the patella engages a smaller, more proximal portion of the trochlea, concentrating load.

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Patella baja. A low patella, on a short patellar tendon, is driven into the trochlea earlier and with greater force during flexion. Patellofemoral contact pressures rise, leading to anterior knee pain and early arthrosis. Knee flexion is restricted because the extensor mechanism tightens early, and the shortened extensor lever arm reduces quadriceps efficiency.

In total knee arthroplasty, baja makes exposure difficult, increases the risk of patellar tendon avulsion during surgery, and can lead to patellar clunk or impingement against the femoral component.

Causes of alta. The patella sits high when the tendon is too long. It may be hereditary or congenital, from a congenitally long patellar tendon (patella alta is often developmental), and growth plate injury at the tibial tubercle (Osgood-Schlatter) can alter tendon length during growth.

Chronic patellar instability is both a cause and a consequence of alta, and chronic habitual dislocation in childhood stretches the extensor mechanism.

Causes of baja. The patella sits low when the tendon shortens or is pulled down. Post-TKA baja is the commonest acquired form, resulting from the level of distal femoral resection, component sizing or positioning, or patellar tendon scarring. Tibial tubercle osteotomy with overcorrected distalisation and patellectomy are further surgical causes.

Scarring and trauma can lower it too. In infrapatellar contracture syndrome (below) it shortens by scarring; trauma can shorten the patellar tendon, or a tibial plateau fracture can displace the tubercle.

The film. The three ratios are measured on a true lateral radiograph at approximately 30 degrees of knee flexion with the femoral condyles perfectly overlapped. The indices assume the patella is imaged in true profile, and an angled or rotated view gives misleading ratios. Tilt shortens the projected patellar articular surface, which inflates any ratio using that surface as its denominator: a rotated lateral radiograph manufactures alta.

Serial films. Flexion angle also changes the measurements, the exception being the Insall-Salvati ratio, which is measured on the patella and its tendon alone. Pre- and post-operative films must therefore be positioned identically; a change in ratio across differently flexed radiographs may be pure artefact.

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The three ratios. Each has different strengths and limitations.

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The ratios are not interchangeable. Hunter and colleagues (2024) showed that the three common indices do not consistently classify the same knee as alta, normal or baja: the Insall-Salvati can label a knee alta while the Caton-Deschamps places it in the normal range, and vice versa. Each ratio measures a different anatomical relationship:

• The Insall-Salvati reflects tendon length relative to patellar size, so a patella with an unusually long diagonal gives a low ratio even if the functional height is normal
• The Caton-Deschamps and Blackburne-Peel reflect functional engagement with the trochlea, but depend on accurate identification of the tibial plateau reference point

In practice and in the exam, pick one index and use it consistently for that patient, including at serial follow-up, and always name the index when communicating patellar height.

The Insall-Salvati, Caton-Deschamps and Blackburne-Peel ratios are the three you must know. When patellar shape or unclear tibial landmarks confound them, a further method or the direct cartilage measure helps.

Indirect versus direct. The radiographic ratios are all indirect: they infer engagement from bony or tendon landmarks. The patellotrochlear index is the direct measure of how much patellar cartilage actually overlaps trochlear cartilage, taken on the midline sagittal MRI section through the thickest cartilage. Because it uses cartilage rather than bone, it detects functional alta that the bony ratios miss.

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Patella alta may be asymptomatic and discovered incidentally, or it may present with patellofemoral instability.

Recurrent patellar dislocation is the hallmark presentation. The patient describes the patella "popping out" to the lateral side, typically in early knee flexion: stepping off a kerb, pivoting, dancing. Alta is one of four major risk factors, alongside trochlear dysplasia, an elevated TT-TG distance and patellar tilt, and multiple risk factors compound the instability risk.

Anterior knee pain. Increased patellofemoral contact pressures from late trochlear engagement cause retropatellar pain, especially on stairs, squatting or prolonged sitting. The pain may be vague and attributed to "chondromalacia" if patellar height is not measured.

J-sign. On active extension from a seated position, the patella may visibly track laterally as it leaves the trochlea in terminal extension. It is a clinical clue to alta, with or without trochlear dysplasia.

Apprehension test. Positive patellar apprehension on lateral displacement in early flexion suggests instability. Combined with a lateral radiograph confirming alta, it strengthens the case for surgical stabilisation that includes distalisation.

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Patella baja typically presents after knee surgery or trauma, with symptoms of extensor mechanism tightness:

• Limited knee flexion - the most common symptom. A post-TKA patient who plateaus at 70 to 90 degrees of flexion despite physiotherapy should have patellar height assessed
• Anterior knee pain - from the patella being driven into the trochlea during flexion
• Extensor lag or weakness - from the shortened lever arm
• Difficulty with stairs and rising from a chair - from limited flexion and reduced extensor power

In severe cases, often post-traumatic or after complex revision surgery, the patient may have a stiff knee with a palpably shortened patellar tendon.

Infrapatellar contracture syndrome is the most important acquired cause of severe patella baja and is asked about directly in the viva. It is an exuberant fibrotic response of the anterior knee soft tissues (the infrapatellar fat pad, the patellar tendon and the peri-patellar retinaculum) after knee surgery, trauma or prolonged immobilisation. The contracting scar tethers the patella distally, producing progressive acquired baja with loss of both flexion and extension.

Prevention is the best treatment: early protected mobilisation after knee surgery and avoiding prolonged immobilisation. A Caton-Deschamps index falling on serial films confirms the progressive baja.

Plain radiographs are first-line. The true lateral described above is the standard investigation for patellar height: the lines for all three ratios can be drawn on it and patellar height calculated directly. The other views add alignment and tracking:

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MRI is the gold standard for assessing the MPFL, trochlear cartilage and patellar articular surface. It can also measure patellar height, applying the same ratios to sagittal sequences, and the TT-TG distance (tibial tubercle to trochlear groove), which guides whether medialisation, distalisation or both are needed. It identifies patellar tendon pathology (tendinosis, partial tears) that may affect the Insall-Salvati ratio.

CT is used for TT-TG measurement in some centres, and for surgical planning in complex cases with prior hardware or malunion.

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Measuring in the viva. Work through the film in order:

1. Confirm the view is a true lateral with condylar overlap, and state the view quality before quoting a number
2. Choose and name the ratio you will use (Insall-Salvati is the default)
3. Identify the landmarks (lower patellar pole, tibial tubercle, patellar diagonal, tibial plateau) and draw the lines
4. Measure, and state the threshold of the index you used
5. Comment on other relevant features (trochlear dysplasia, patellar tilt, osteoarthritis) and relate the finding to the clinical picture

Untreated alta. Left alone, a high-riding patella risks:

• Recurrent patellar dislocation, with cumulative damage to the patellar articular surface and the MPFL
• Patellofemoral osteoarthritis from chronic instability episodes, abnormal tracking and increased contact pressures
• Psychological impact and activity restriction from fear of dislocation
• Quadriceps wasting and deconditioning from activity avoidance

Untreated baja. Left alone, a low-riding patella risks:

• Progressive stiffness with permanent loss of knee flexion
• Patellofemoral arthrosis from chronically elevated contact pressures
• Functional disability: difficulty with stairs, sitting and driving
• Patellar tendon rupture - rare, in severe cases with chronic tendon shortening and degeneration

Surgical treatment. Each procedure has its own complications:

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• No single international guideline exists specifically for patellar height measurement or correction. The Insall-Salvati, Caton-Deschamps, and Blackburne-Peel ratios are the universally accepted radiographic indices, taught across all training programmes globally.
• Patellar instability management (which incorporates patellar height correction) is addressed by the International Patellofemoral Study Group (IPFSG) consensus statements and in the patellofemoral instability guidelines published by various national orthopaedic societies (including AAOS, BOA/BOAST, and DGOOC). The consensus is that patella alta should be assessed and corrected when significant in the setting of recurrent instability, typically as part of a tibial tubercle osteotomy with distalisation.
• TT-TG distance thresholds vary slightly between centres but the general consensus is: normal approximately 10 to 15 mm; surgical correction considered above 20 mm. The threshold may differ between CT-based and MRI-based measurements (CT values tend to be higher by 2 to 4 mm).
• Post-TKA patella baja is increasingly recognised as an outcome measure in arthroplasty registries. The National Joint Registry (NJR, UK) and Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) report on patellofemoral complications and revision rates. Joint line preservation during TKA is the key preventative measure and is emphasised in surgical technique guides from all major implant manufacturers.
• Global practice variation exists in the choice of patellar height index: the Insall-Salvati remains the most widely used in examinations and initial assessment, while the Caton-Deschamps is increasingly preferred for pre-operative planning of distalisation surgery in Europe. In Asian countries where patellofemoral instability surgery is common, the modified Insall-Salvati and CD indices are both used with population-specific normative data that may differ from Western reference ranges.