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. When the patella sits too high it is called patella alta; when it sits too low it is patella baja. Both are important because they alter the biomechanics of the patellofemoral joint and have specific clinical consequences.

Patella alta is the more common abnormality and is a well-established risk factor for recurrent patellar instability. The high-riding patella does not engage the trochlear groove until later in the flexion arc, meaning that through the range where most daily activities occur the patella is poorly constrained. Patella baja is less common but is an important iatrogenic problem after knee surgery - particularly total knee arthroplasty and tibial tubercle osteotomy - and causes restricted flexion, anterior knee pain, and difficulty with future revision surgery.

Three radiographic ratios are used to classify patellar height: the Insall-Salvati, Caton-Deschamps, and Blackburne-Peel indices. They are measured on a true lateral radiograph and each has different strengths and limitations - critically, they are not interchangeable, and a patient classified as alta by one may be normal by another.

For the exam, three themes recur: how to measure patellar height accurately (which view, which ratio, and the thresholds), what patella alta and baja mean clinically (instability versus stiffness), and what to do about them (when and how to correct patellar height surgically).

Normal patellofemoral biomechanics

The patella acts as a sesamoid bone within the quadriceps extensor mechanism, increasing the moment arm of the quadriceps by up to 30 percent and distributing compressive forces across the femoral trochlea. During knee flexion, the patella tracks within the trochlear groove, engaging the groove at approximately 20 to 30 degrees of flexion and deepening its congruence as flexion increases.

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

Patella alta: biomechanical consequence

When the patella sits too high (a long patellar tendon), it enters the trochlear groove later in the flexion arc - typically beyond 30 to 40 degrees rather than at 20 degrees. This means that through the first 30 to 40 degrees of flexion, the patella is perched above the groove and relies almost entirely on soft-tissue restraints (principally the MPFL) for stability. The practical consequence is a significantly increased risk of lateral patellar dislocation, especially in early flexion or with a valgus and external rotation force.

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

Patella baja: biomechanical consequence

When the patella sits too low (a short patellar tendon), the patella is driven into the trochlea earlier and with greater force during flexion. This causes:

• Increased patellofemoral contact pressures, leading to anterior knee pain and early arthrosis
• Restricted knee flexion because the extensor mechanism tightens early
• Shortened extensor lever arm, reducing quadriceps efficiency

In total knee arthroplasty, patella 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.

Patellar height is classified using radiographic ratios measured on a true lateral knee radiograph taken at approximately 30 degrees of knee flexion with the femoral condyles perfectly overlapped. The correct technique is critical because rotation and flexion angle change the measurements.

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The non-interchangeability problem

A landmark study (Hunter et al., 2024) demonstrated that the three common patellar height indices do not consistently classify the same knee as alta, normal, or baja. The Insall-Salvati ratio can label a knee as alta while the Caton-Deschamps places it in the normal range, and vice versa. This happens because each ratio measures a different anatomical relationship:

• The IS ratio reflects tendon length relative to patellar size - so a patella with an unusually long diagonal will give a low IS ratio even if the functional height is normal
• The CD and BP ratios reflect functional engagement with the trochlea - but depend on accurate identification of the tibial plateau reference point

For clinical practice and exam answers, the important principle is: pick one index and use it consistently for that patient, and always name the index when communicating patellar height.

The Insall-Salvati, Caton-Deschamps and Blackburne-Peel ratios are the three you must know, but a candidate who can name a further method or the direct cartilage measure scores well, especially when the standard ratios are confounded by patellar shape or unclear tibial landmarks.

Patella Alta

Patella alta may be asymptomatic and discovered incidentally, or it may present with features of patellofemoral instability:

Patella Baja

Patella baja typically presents after knee surgery or trauma and causes symptoms related to extensor mechanism tightness:

• Limited knee flexion - the most common symptom. The tight extensor mechanism prevents deep bending. Post-TKA patients who plateau at 70 to 90 degrees of flexion despite physiotherapy should have patellar height assessed.
• Anterior knee pain - increased patellofemoral contact pressures from the patella being driven into the trochlea during flexion.
• Extensor lag or weakness - a shortened lever arm reduces quadriceps mechanical advantage.
• Difficulty with stairs and rising from a chair - functional impairment 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 together with loss of BOTH flexion and extension.

The best treatment is prevention: early protected mobilisation after knee surgery and avoiding prolonged immobilisation. The Caton-Deschamps index falling on serial films confirms the progressive baja, and management is matched to the phase - aggressive physiotherapy and anti-inflammatories while the process is active, and surgical release reserved for the burnt-out stage.

Plain radiography (first-line)

The true lateral knee radiograph at approximately 30 degrees of flexion is the standard investigation for patellar height. The femoral condyles should overlap perfectly; any rotation introduces error.

On the lateral view, the examiner can draw the lines for the Insall-Salvati, Caton-Deschamps, and Blackburne-Peel ratios and calculate patellar height directly.

Additional views:

Cross-sectional imaging

• MRI is the gold standard for assessing the MPFL, trochlear cartilage, and patellar articular surface. It can also measure patellar height (the same ratios applied to sagittal MRI sequences) and the TT-TG distance (tibial tubercle to trochlear groove), which guides whether medialisation, distalisation, or both are needed. MRI also 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.

Measurement workflow for the exam

1. Confirm the view is a true lateral with condylar overlap
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)
4. Measure and state the threshold (IS: normal 0.8 to 1.2, alta above 1.2, baja below 0.8)
5. Comment on other relevant features (trochlear dysplasia, patellar tilt, osteoarthritis)

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Patella Alta

Management depends on whether the alta is symptomatic and whether instability is the presenting problem.

Tibial tubercle distalisation

When patellar height correction is indicated, the standard approach is a tibial tubercle osteotomy with distal transfer (often combined with medialisation to correct an elevated TT-TG distance):

• An osteotomy of the tibial tubercle is performed (usually a flat or step-cut)
• The tubercle fragment is slid distally by the calculated amount (based on the ratio excess above normal)
• Fixed with screws in the new position
• The Caton-Deschamps index is often preferred for planning the amount of distalisation because it directly measures the functional relationship

Important: overcorrection to patella baja must be avoided - aim for a normal CD index (around 1.0), not the lowest possible position. The amount of distalisation is calculated pre-operatively from the measured index.

Patella Baja

Complications of untreated patella alta

• 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

Complications of untreated patella baja

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

Complications of surgical treatment

Patellar height assessment is a practical skill that appears across multiple exam stations. In the radiology viva, you may be handed a lateral knee X-ray and asked to measure and interpret patellar height - the key is a systematic approach: name the ratio, draw the lines, apply the thresholds, and relate the finding to the clinical picture. In the clinical viva, patella alta is an integral part of the patellar instability workup alongside trochlear dysplasia, TT-TG distance, and patellar tilt - knowing how these factors interact to determine the surgical plan is essential. In the basic science viva, the biomechanics of the patellofemoral joint and how patellar height affects trochlear engagement and contact pressures links anatomy to clinical decision-making.

Patella baja is most often encountered in the revision TKA setting where it complicates exposure and limits outcomes - knowing the strategies for dealing with a shortened extensor mechanism (tibial tubercle osteotomy, quadriceps snip, V-Y turndown) is a common exam question.

The key message for the exam: patellar height is a measurable, correctable risk factor that should be assessed in every patient with patellar instability and every post-TKA patient with limited flexion. Know the ratios, their thresholds, and their limitations.

• 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.