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All Things Patella


Patella dislocation

Robert is a 14-year-old boy who has just arrived by ambulance having been playing a rugby match. He was running just short of the try line when he tried to make a dastardly last-minute course correction, however, while rapidly altering course his body went one way, while his foot remained planted on the ground and he felt his left knee suddenly give way. A sudden surge of pain followed and he dropped to the ground. On the ambulance stretcher, you see his knee is hugely swollen with an obvious deformity out laterally. He is in obvious pain and distress. The triage nurse thinks his patella is dislocated and wants you to prescribe some analgesia.     


Patella dislocation is a common knee injury, particularly associated with sports (61-72% Steiner 2010) and physical activity among teenagers.



The two most common mechanisms for a patellar dislocation are:

  • Non-contact twisting injury (66-82% Khormaee 2015) – this is where foot remains planted on the ground, usually externally rotated, while the knee is extended and internal rotation about the hip causes a dislocation.
  • Direct contact (less common) – e.g. knee to knee strike during basketball or a helmet/head to knee in rugby.


Risk factors

Some children are more prone to patellar dislocation than others. It is worth noting that those with underlying anatomical abnormality may not present with as much obvious swelling and deformity as those with normal anatomy. Look out for the following:

  • Connective tissue disorders e.g. Ehlers-Danlos
  • Lateral patellar tilt
  • Trochlear dysplasia
  • Genu valgum (‘knock-knee’)
  • Patella alta (high-riding patella)
  • Increased femoral anteversion
  • Vastus medialis muscle hypoplasia



A child will usually present following a sudden ‘pop’ or sensation of instability and severe anterior knee pain. Acute dislocation is usually associated with a moderate haemarthrosis, however, those with underlying risk factors (especially ligament hyperlaxity) may not.

A detailed ligamentous exam is important to assess for integrity and damage to cruciate and collateral ligaments. Medial tenderness is common as the MPFL (medial patella-femoral ligament) is ruptured in over 94% of dislocations. Providing over half of the restraining force for the patella, it extends to its medial border from the femur.

The patellar apprehension test is described for those whose dislocations have reduced pre-hospital (according to Willis et al. most spontaneously reduce). With the knee flexed to 30 degrees, apply some lateral pressure; with medial instability, the patient will feel apprehensive about the kneecap “popping” again.



This is a painful injury so ensure adequate analgesia has been provided. Next question – has the dislocation been reduced yet? If yes – jump to post-reduction management, if not read on…

The use of procedural sedation with nitrous oxide is ideal for this short painful procedure. Flex the hip on the affected side, thereby relaxing the quads muscle. Apply pressure to the lateral border of the patella in a medial direction while extending the knee. A satisfying ‘clunk’ should be felt as the patella slides back into its home in the trochlear groove of the distal femur.


Post-reduction management:

There is no evidence-based consensus to guide ongoing treatment, especially in first-time dislocations (see controversies below). Once reduced, the application of a knee immobilizer will help reduce ongoing analgesia requirements. Studies have failed to demonstrate a benefit of one type of immobilization device over another – therefore follow local guidance; knee brace in full extension, cylinder cast or above knee back-slab are all acceptable.

Ensure ongoing analgesia requirements post-discharge are met, and consider crutches until seen back in the orthopaedic clinic.



This is not always necessarily required pre-reduction. A post-reduction x-ray (AP and lateral) is important to assess associated osteochondral fractures and to check the patella location. An MRI scan can be particularly useful to assess for associated ligamentous integrity and rupture, VMO sprain, and osteochondral fractures. In fact, MRI is more sensitive than arthroscopy to assess for MPFL tear. However, this can be organized from orthopaedic follow-up clinic.


What to tell the patient

Will it happen again?

The younger the patient the higher the rate of re-dislocation: 60% for those 11-14 years, and 33% 15-18 years.

Is everything fixed now that the kneecap is back in place?

It can be difficult to tell is there are associated ligamentous injuries on the day of presentation which can lengthen recovery to baseline. Not all osteochondral fractures will be apparent on the plain film radiograph. Repeat examination in the orthopaedic clinic +/- MRI will often provide a more detailed assessment of prognosis.

What happens next?

Traditionally first-time dislocations were treated with immobilization for 3-6 weeks followed by intensive physio to strengthen the quads. There is now a move towards earlier motion and rehab (despite a lack of RCTs). Surgery has usually been reserved for those requiring loose body (within the joint) removal, fixation of osteochondral fractures and for those with recurrent instability and dislocation.

When can I go back to playing sports?

As a general rule of thumb 8-12 weeks.



There has been a recent trend towards surgical fixation (usually of the MPFL) in recent times. Whereas this was usually reserved for recurrent dislocations, a systematic review by Nwachukwu et al in 2015 showed reduced re-dislocation with surgical treatment of first time patellar dislocation (31% Vs 21%, p=0.04). However, there was no difference between surgery versus conservative management in subjective or objective knee function.

Why is this important? Ultimately it will depend on local orthopaedic preference but it’s important to know that not all first-time dislocations will be treated conservatively so we don’t inadvertently give parents and patients misinformation.


Case Resolution

You correctly identify this as a patellar dislocation and organize a nitrous oxide procedural sedation and successfully reduce the dislocation. Post-procedure you place his knee in an above-knee backslab and organize a fracture clinic follow up. Before leaving you to ensure his parents have appropriate dosed analgesia at home.


Patellar tendon rupture

Rachel is a 13-year-old girl who is a keen runner and has recently discovered a passion for hurdling. She was attending training, on her third lap, while jumping over the hurdle felt a sudden pop followed by immense pain causing her to drop to the ground. She was unable to walk, the pain being too intense. An ambulance was called and the triage nurse asks you to see her next as she is crying in pain.


This is a relatively rare condition with the peak age of occurrence being 40. That being said, the rate of patella tendon rupture (PTR) is increasing in frequency. This is often a sports-related injury (e.g. hurdling, basketball) with the mean age for children being 13 years.



The patellar tendon is part of the very important extensor mechanism of the knee, connecting the patella to the tibial tuberosity. This is crucial to help us overcome the forces of gravity. The entire mechanism includes the quads femoris muscles, the quads tendon, the patellar tendon, the patella itself and the tibial tubercle.

The most common ways to injure the PTR is through a direct blow (e.g. fall), or through forceful contraction of the quads muscle (usually while the foot is planted and knee flexed) – e.g. missing a step while climbing stairs, or in jumping sports.

The force required to rupture the tendon is 17 times that of the average body weight

There are three main patterns of injury which can result

  • Avulsion from the inferior pole of the patella (there is an increased risk of concurrent bony avulsion with children’s growth plates)
  • Mid-tendon
  • Distal avulsion from the tibial tuberosity


Risk factors

Conditions that cause microscopic damage to the tendon blood supply such as repeated microtrauma (e.g. athletes), chronic renal failure, collagen vascular disease, diabetes, osteogenesis imperfecta, and steroid use.



There will usually be some history of a popping sensation, knee pain, swelling and difficulty or inability to weight bear. On exam, the knee usually has a moderately large haemarthrosis. A high riding patella (patella alta) may be noted when compared to the contralateral side.  Localized tenderness and a palpable gap below the inferior pole of the patella may be present in complete tears. The pain will limit the range of movement of the knee. The inability to actively straight leg raise is indicative of serious extensor mechanism pathology and should be assumed to be a complete tear until proven otherwise. Pain may limit the range of movement examination of the knee (ensure adequate analgesia) so an alternative is to have the patient maintain a passively extended knee.



X-rays – AP and lateral. While not conclusive, certain features may suggest the diagnosis. A joint effusion is likely, the presence of a high riding patella (patella alta) is highly suggestive of a complete tear.


Want to really impress your orthopod service – measure the Insall-Salvati ratio and if >1.2 – this is diagnostic of patella alta. This is measured by A/B, where A= patellar tendon length (posterior surface of the tendon from the lower pole of the patella to tibial insertion) and B = patellar length (longest pole -> pole length).

Ultrasound – while this modality is user and operator dependent, its availability aids its usefulness. It can be effective at detecting and localizing disruption to the tendon. Differentiating between partial and complete tears can be more challenging.


MRI – the favoured imaging modality, but access can limit its usefulness. It will aid accuracy in delineating partial from complete tears and reveal any associated bony avulsion or soft tissue injuries.



Consider the diagnosis. While a rare event among the paediatric population, delayed diagnosis causes increased morbidity. 7% of those who sustain acute trauma to the knee will have a PTR. These are painful injuries, and so judicious use of analgesia is paramount.


Conservative management may be considered for those children with only partial tears and an intact extensor mechanism. Immobilization with a removable knee splint in full extension followed by graduated weight-bearing and rehab program.


Surgical repair is indicated for complete tears. A comparison of different techniques is difficult due to the small numbers of patients. Two main options exist depending on the level of the tear, associated injuries and surgeons choice:

  • Primary repair – using end to end repair, trans-osseous repair or suture anchor tendon repair.
  • Tendon reconstruction is usually reserved for severely disrupted tendons and involves the use of an autograft.



Traditional management post-surgery involved applying a cylinder cast for six weeks allowing the child to weight bear as tolerated. A newer school of thought involved early controlled movement at the joint. This involves applying a knee brace which allows up to 90 degrees of flexion for four weeks followed by graduated controlled increases in flexion until 12 weeks. This early mobilization aids quicker knee function return and prevents muscle atrophy.


What to tell the patient

Will I need an operation?

If there is complete tendon rupture or a compromised extensor mechanism, then surgical repair is needed.

When can I return to sports?

Full return to sports usually takes six months (range of 13-30 weeks depending on exact injury).


Case Resolution

On examination, you note she Rachel is unable to straight leg raise. An x-ray shows a large haemarthrosis. You refer her to the on-call orthopaedic service and the next day she undergoes an MRI confirming your suspicions of a patellar tendon rupture. This is operatively repaired and she has now commenced her rehab, looking forward to returning to running soon.


Patella fracture

The next card you pick up is a 14-year-old boy (Brian) who is a ‘return’ patient. He attended two days ago with right knee trauma and has represented with ‘ongoing pain’. He was playing rugby at the time and made a sudden twisting movement while avoiding a tackle. He felt searing hot pain in his knee and dropped to the ground. He wasn’t able to walk and had to be stretchered off the pitch. He had intranasal fentanyl pre-hospital and a top-up dose in triage two days ago. Being a keen sports player and with the rugby final coming up he was determined to return to play and was documented in the notes as being able to ‘tentatively weight bear’. On exam, you note a tense, swollen right knee. Being stoic he denies any focal tenderness but you notice a grimace when you examine his inferior patella. He is unable to straight leg raise and when he attempts to weight bear fully he is clearly in pain. You wonder whether his patellar tendon might be injured as surely a fracture would have been noted on his XR from two days ago….


Patellar fractures are relatively rare, with an incidence of 0.5-1.5% of all skeletal injuries. They are most common between 8-16 years with a mean age of 12.4 years. Unsurprisingly they are mainly caused during sporting and leisure activities.



There are two main types of patellar fractures in paediatrics. The first is a ‘typical’ bony fracture, which is caused by a direct impact on the patella, similar to adults. The second is almost exclusive to paediatrics; patellar sleeve fracture. These are caused by indirect trauma to the knee and are the result of a forceful quads muscle contraction in a skeletally immature individual. Patellar sleeve fractures are three times more common in males than in females. They account for over 50% of all patellar fractures in children. Inferior pole sleeve fractures are most common.



There may be a history of either a direct blow to the knee or a non-contact twisting injury with the foot planted. A knee effusion is likely to be present, with point tenderness at the affected site. Inability to extend the knee fully and inability to fully weight bear are red flags and underlying pathology must be assumed. Patella alta or patella baja may be present with patellar sleeve fractures with disruption of the inferior (alta) or superior (baja) tendon. A palpable gap may be present inferior/superior to the patella with complete disruption of the corresponding tendon.



X-ray: AP and lateral radiographs. A haemarthrosis may be visible. Bony fractures of the patella are most likely to be transverse or vertical. The patellar sleeve fractures require a high index of suspicion and can easily be missed as often only a tiny sliver of avulsed bone can be seen on plain film. Don’t be misled by this seemingly innocuous x-ray finding, a significant cartilaginous component will be involved.

Image from Orthobullets: Patellar sleeve fracture

Up to 8% of the population have bipartite patella – don’t confuse this for a patella fracture. Contralateral imaging is not always useful as 50% of those affected will have bilateral patella affected. Characteristically it will be present supero-laterally and will have smooth edges around the cortex.

Courtesy of Orthobullets: X-Ray showing bipartite patella

Ultrasound: can be helpful in confirming injury to the tendon. Soft tissue oedema and fluid hyperemia are other indirect signs that may be seen suggestive of tendon disruption.


MRI: this remains the imaging modality of choice if the diagnosis is in doubt. This will allow displaying the extent of the chondral injury and any concomitant extensor mechanism injury.


Non-operative: this may be considered in those with a nondisplaced (<2mm) fracture and an intact extensor mechanism. A cylinder cast will be applied for six weeks and then intensive rehab following its removal.


Operative: Any displaced fracture (>2mm) usually require open reduction and internal fixation (as the extensor mechanism needs to be restored).

Patella sleeve fractures will require ORIF with suturing of the tendon; superior sleeve fracture necessitating quads tendon repair, and inferior sleeve fracture requiring patellar tendon repair.

Post-operatively these patients are usually placed in an extension brace or cast until the wound is healed and then active flexion and extension exercises are commenced to restore normal knee function as soon as possible.


What to tell the patient?

Is there a fracture there?

If no fracture can be seen but the child has a large swollen knee, cannot weight bear or cannot fully extend the knee – have a low threshold to immobilize the knee even in the absence of a fracture on plain film. Patellar sleeve fractures are easily missed and lead to increased morbidity for the patient. If ready access to ultrasound or MRI this will confirm or refute the diagnosis, otherwise immobilize the knee and arrange confirmatory imaging as soon as possible or via local orthopaedic outpatients.

When can I return to sport?

Depending on the fracture and associated tendon injury – usually 3-6 months.


Case resolution

On review of his x-ray from two days previously you notice a tiny sliver of bone inferior to the patella and you organize an ultrasound which confirms your suspicions of a patellar sleeve fracture, involving his patella tendon. You apologize to his parents for the delayed diagnosis and refer him to the on-call orthopaedic service for an ORIF.




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  • Top marks if you can spell AND pronounce my name correctly – even my mother misspelled my name on the birth cert! Adrenaline Junkie who favours some emergency medicine on the side. Special interest in PEM and critical care. When I’m not putting stickers on kids and waving unicorn bubble wands around, I can be found out adventuring; running, cycling, kayaking and hiking. Love: sparkling water, owls, unicorns. Dislike: people spelling my name incorrectly!


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