Illustration showing how a torn medial patello-femoral ligament (MPFL) can allow the kneecap to dislocate to the lateral (outer) side.
Illustration showing how a torn medial patello-femoral ligament (MPFL) can allow the kneecap to dislocate to the lateral (outer) side.
What happens during a patellar dislocation?
In the first dislocation event the patella usually dislocates to the lateral (outer) side.
This is due to the anatomy of the region. At the same time other structures may be injured, such as the medial patellofemoral ligament which may be torn, and there may also be a small fracture on the medial side of the patella and some associated bone bruising.
Peer-reviewed papersQuote from peer-reviewed paper:
"....First patellar dislocation is a common injury of the knee, involving often adolescents and the active population. The consequences of the first episode can be various and potentially disabling. Among these, acute patellar dislocation can often result in recurrent patellar instability....."
Citation: Bulgheroni E, Vasso M, Losco M, Di Giacomo G, Benigni G, Bertoldi L, Schiavone Panni A. Management of the First Patellar Dislocation: A Narrative Review. Joints. 2019 Dec 31;7(3):107-114. doi: 10.1055/s-0039-3401817. PMID: 34195538; PMCID: PMC8236325.
Quote from peer-reviewed paper:
"....Soccer, gymnastics, and dancing are typical injuries, where outward rotational forces and valgus stress when flexing a knee may drive the patella out of the sulcus. In rare cases, dislocation may be due to a direct blow to the medial aspect of the patella.
Citation: Sinikumpu J, Nicolaou N. Current concepts in the treatment of first-time patella dislocation in children and adolescents. J Child Orthop. 2023 Feb;17(1):28-33. doi: 10.1177/18632521221149060. Epub 2023 Jan 12. PMID: 36755554; PMCID: PMC9900011.
What predisposes to patellar dislocation?
A number of conditions predispose a person to patellar dislocation:
- patella alta - when the kneecap rides too high
- trochlear dysplasia - when the underlying groove is abnormal
- increased TT-TG distance - when the anatomy leads to increased sideways force on the patella
- patellar tilt - when tissue imbalance leads to a tilted patella, and tightening of the tissues at the side of the knee aggravated it
Quote from peer-reviewed paper:
Acute patellar dislocation ...."is the result of an indirect force on the knee joint, which leads to valgus and external rotation of the tibia relative to the femur. PD is unlikely to occur on a knee with normal patellofemoral joint (PFJ) anatomy....."
Usually there is "....an acute injury of the ligamentous medial patellar stabilizers in the background of factors predisposing to patellar instability....
Citation: Samelis PV, Koulouvaris P, Savvidou O, Mavrogenis A, Samelis VP, Papagelopoulos PJ. Patellar Dislocation: Workup and Decision-Making. Cureus. 2023 Oct 9;15(10):e46743. doi: 10.7759/cureus.46743. PMID: 38021800; PMCID: PMC10631568.
Why can't I bend my knee after dislocating it?
During a patellar dislocation, the kneecap jumps out of the underlying groove in which it usually glides.
If the restraining MPFL is torn, it may not return to the groove after the event, but may remain caught on the other side with the knee bent, and attempts to straighten the knee will be very painful.
An anaesthetic with muscle relaxation may be needed to allow the clinician to return the kneecap to the groove, which will allow the leg to be straight again.
Can a dislocated knee be permanent?
With proper management at the time, it should always be possible to return the kneecap to the groove.
A delay in treatment with poor underlying anatomy may lead to a permanent disfigurement.
Peer-reviewed papersQuote from peer-reviewed paper:
"....The percentage of recurrent patellar dislocation after the first episode varies from 15 to 80%,....."
Citation: Bulgheroni E, Vasso M, Losco M, Di Giacomo G, Benigni G, Bertoldi L, Schiavone Panni A. Management of the First Patellar Dislocation: A Narrative Review. Joints. 2019 Dec 31;7(3):107-114. doi: 10.1055/s-0039-3401817. PMID: 34195538; PMCID: PMC8236325.
Management of patellar dislocation
A first event may be managed conservatively, with a period of immobilisation, after which sports may be limited for some months.
If there is a subsequent event, the patient is usually subjected to a full investigation of any underlying causes and surgery may be planned. This may include a reconstruction of the MPFL. In patients where there is a marked problem with the anatomy, such as trochlear dysplasia, corrective surgery may be undertaken.
Peer-reviewed papersQuote from peer-reviewed paper:
"....there is no evidence that bone abnormalities should be corrected after the first episode of patella dislocation, although in rare cases of severe associated anatomic patellofemoral deformities, procedures like tibial tuberosity transpositions and/or trochleoplasty can be taken into consideration and associated to MPFL repair/reconstruction...."
Citation: Wolfe S, Varacallo M, Thomas JD, Carroll JJ, Kahwaji CI. Patellar Instability. 2023 Sep 4. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. PMID: 29494034.
Forum discussions
- "left knee: patella alta+former dislocation--> incessant pain, also neuropathic"
Patients discuss ongoing pain after patellar dislocation.
- Advice regarding Distal Femoral Osteotomy vs MPFL reconstruction and return to activity
Patients discuss restoring the anatomy to prevent future patellar dislocations.