What’s the correct management for someone with a 1st-time dislocation of their patella?

 

The answer is easy and really quite indisputable: the answer is send them for imaging!

 

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Risk of Redislocation After Primary Patellar Dislocation: A Clinical Prediction Model Based on Magnetic Resonance Imaging Variables.
Arendt EA, Askenberger M, Agel J, Tompkins MA.
Am J Sports Med2018; 46(14): 3385-3390

 

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In recent years there’s been an increasing number of papers published looking at whether or not early surgery is appropriate for someone presenting with their first ever patellar dislocation. Early surgery is good if it prevents further dislocations; however, surgery carries with it the risks of potential infection, neurovascular damage, blood clots etc, as well as causing pain, hassle, time off work etc etc.

It has in the past been quoted that the risk of a 1^st-time patellar dislocater dislocating again, subsequently, is only about about 17% [Fithian]. However, Arendt et al.’s recent study showed a 42% rate of recurrent dislocation within 2 years after primary dislocation [Arendt]. Even if Fithian’s figures are the more accurate of the two, one could still argue that a majority of first-time dislocations do notend up dislocating again. This could be put forward as an argument to support the conservative management of all first-time dislocaters.

The problem with recurrent dislocation is that it leads to progressive attenuation of the medial patellofemoral ligament and medial retinaculum. In addition, increasing apprehension leads to fear avoidance, which leads to muscle inhibition with wasting and weakness (particularly of the VMO), which further increases the risk of recurrent dislocation and a downwards spiral. Furthermore, importantly, every time a patella dislocates there’s a risk of progressive articular cartilage damage within the patellofemoral joint, which significantly increases the long-term risk of future patellofemoral arthritis.

So, after initial consideration, it seems easy: conservative management with intensive physio rehab for a first-time dislocater and potential surgical stabilisation for a recurrent dislocator. Easy… but wrong!

First of all, when someone dislocates their patella, the medial side of the patella tends to impact hard against the lateral edge/side of the lateral femoral condyle. This causes a typical pattern of bone bruising on MRI; however, the main concern is what potential articular cartilage damage might have been caused within the patellofemoral compartment. If the patient with a recent dislocation has a chunk of loose cartilage / bone broken off within the joint and / or areas of unstable articular cartilage damage, then these are best treated surgically, arthroscopically, to remove any loose bodies and to stabilise the articular cartilage damage by radiofrequency chondroplasty. [LINK]

How are you going to know whether or not the patient does have significant articular cartilage damage? The answer is simple: the patient needs an MRI scan! (preferably on a high-res 3T scanner).

So, for this reason alone, the answer to the question of how one should manage a first-time patella dislocater is simple: send them for imaging.

Not all people are the same! Most studies take cohorts of patients with a particular disease or diagnosis and compare the outcomes of different treatments for those patients. However, there’s an assumption here that all the patients in each cohort are broadly the same. Researchers always check comparative cohorts for basic demographics such as age, sex etc., but these really are just the absolute basics. Are all people’s knees the same and do all people have similar risks for instability? The answer is a very obvious ‘no’!

The recent paper by Arendt et al. highlights elegantly how certain risk factors can be identified from MRI that can help predict the risk of future recurrent dislocation. These risk factors include:

• Age: the younger the patient is, the greater the risk of further dislocations.
• Sex: the risk of further dislocations is higher in females.
• The magnitude of the trauma: the lower the energy of the accident where the patellar dislocation occurred and the more innocuous the injury, the more likely it is that the patient had an inherent predisposition to instability.
• History of previous dislocations in that same knee: the more times one dislocates one’s patella, the more likely it is that the patella will simply dislocate again.
• Patellofemoral dysplasia [LINK]
• Patella alta [LINK]
• Patellar maltracking / increased TT-TG distance [LINK]

 

 

Parikhet al [Parikh] have recently taken 4 of the main risk factors for recurrent dislocation and created a very easy algorithm to aid in the prediction of recurrent instability. These risk factors are:

1. Skeletal immaturity
2. History of contralateral dislocation
3. Trochlear dysplasia
4. Patella alta

Their recommended algorithm is:

Risk Factors	Average predicted risk of recurrence	Treatment recommendation
0	13.8%	Conservative treatment
1	30.1%	Conservative treatment
2	53.6%	Surgery optional
3	74.8%	Surgical treatment
4	88.4%	Surgical treatment

Similarly, in Arendt et al.’s study, three specific factors were highlighted as factors that contribute to an increased risk of further dislocations, being:

1. skeletal immaturity,
2. trochlear dysplasia (an increased sulcus angle), and
3. patella alta (a reduced Insall Salvati ratio).

They calculated that the risk of further patellar dislocation after first-time dislocation correlated very strongly with the number of risk factors present:

• 0 risk factors present = 5.8% risk of further dislocations
• 1 risk factor present = 22.7% risk of further dislocations
• 3 risk factors present = 78.5% risk of further dislocations!

Importantly, for one to be able to make these important predictions one needs to be able to identify and accurately measure each of the specific individual risk factors… in particular, one needs a full appreciation of the morphological characteristics of the patient’s patellofemoral joint, and in particular trochlear morphologyand patellar height. These factors can only be assessed and measured via appropriate imaging.

So, once again, appropriate imaging is key.

Only by obtaining appropriate imaging can one A) exclude significant patellofemoral articular cartilage damage, B) delineate the exact patellofemoral anatomy/morphology, and thus C) define the possible risk factors for potential recurrent dislocation and therefore D) advise the patient properly about the most appropriate way forward, in terms of treatment.

In my practice, unless there’s a chondral loose body or really severe/unstable articular cartilage damage, then the very large majority (near all) of first-time patella dislocaters will be managed at least initially conservatively, with appropriate physiotherapy rehab. However, if you don’t look properly then sooner or later you’re going to miss something important, and you can’t give a full and measured opinion without first having all the necessary information. Therefore, the answer to the question as to how best to manage a first-time patellar dislocater is simple: all patients should be referred to a specialist knee surgeon for a full assessment and for imaging prior to a fully informed discussion then being had which then guides the decision-making in terms of actual treatment. Don’t get me wrong, imaging is not the be-all and end-all, and indeed you treat the patient, not the ‘picture’. However, appropriate imaging is essential for the clinician to have the full picture and all the necessary information to be able to make a safe and appropriate informed decision with respect to how best to then advise the patient about the most appropriate treatment options for their individual knee.

 

Conclusions

• Any patient sustaining a patellar dislocation must be sent for an MRI scan ASAP (preferably on a high-res 3T scanner).

• If the patient hasn’t already previously had further patellofemoral imaging, then they should also be sent for X-rays with:

1. a weight-bearing AP view,
2. a lateral view and
3. a patellar skyline view.

• If you want to see the patellofemoral anatomy really clearly, with the best possible views, then you should also get a fine-cut CT scan with 3D reconstruction and with special trochlear views (with the tibia and the patella removed, and with the femur flexed to 30 degrees).

• You shouldn’t make decisions about patient treatments without actually fully involving the patient themselves in the decision-making process, and you can’t give informed consent to treatment without being fully informed… and you can’t be fully informed unless you’ve got the full information….. and you can’t possibly have the full information without having the full appropriate imaging.

 

READING

 

Arendt EA, Askenberger M, Agel J, Tompkins MA. Risk of Redislocation After Primary Patellar Dislocation: A Clinical Prediction Model Based on Magnetic Resonance Imaging Variables. Am J Sports Med 2018; 46(14): 3385-3390. LINK

 

Fithian DC, Paxton EW, Stone ML, et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med 2004;32(5):1114–1121. LINK

 

Parikh SN, Lykissas MG and Gkiatas I. Predicting Risk of Recurrent Patellar Dislocation. Curr Rev Musculoskelet Med2018 Jun; 11(2): 253–260.LINK

 

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Written by:

Mr Ian McDermott  MB BS, MS, FRCS(Orth), FFSEM(UK)
Consultant Knee Surgeon, London Sports Orthopaedics
Honorary Professor Associate, Brunel University
www.kneesurgeon.london
www.sportsortho.co.uk