An unusual case masquerading as lateral patellar instability

A 27-year-old female patient presented with a 12-month history of recurrent anterior knee pain, instability and a ‘clicking’ sensation aggravated by activity, with no preceding traumatic injury.  The patient had tenderness overlying the lateral border of the patella and the remainder of the clinical examination was normal, including a negative patellar apprehension test and absent ‘J’ sign.

Presentation MRI:

• Increased signal within the superolateral aspect of Hoffa’s fat pad on PD FS and T2 sequences, attributed to oedema secondary to patellar mal-tracking (Figure 1).
• No contusions to the medial patella or lateral femoral condyle to suggest recent lateral patellar dislocation.
• Normal medial patellofemoral ligament.
• No features of trochlear dysplasia or patella alta. 
• Normal tibial tuberosity – trochlear groove distance of 10 mm.

There was no clinical improvement after 8 months of conservative management with physiotherapy and repeat MRI was performed to re-assess for a surgical target:

• Marked progression of the lesion within Hoffa’s fat pad, which had increased in size and was now more mass-like, with low signal intensity foci on PD FS and T2 sequences (Figure 2A and 2B).

• Gradient echo imaging was performed, which demonstrated blooming artefact within the lesion in keeping with haemosiderin deposition and localised pigmented villonodular synovitis (PVNS) (Figure 2C).
• The mass was relatively laterally subluxed on the gradient echo sequences when compared with the earlier PD imaging (Figure 2).

This case is an example of a rare pathology, PVNS, manifesting with common mechanical symptoms of lateral patellar instability.  PVNS is a benign proliferative condition affecting the synovial lining and both diffuse and localised patterns have been described [1]. The aetiology remains unclear, however cytogenetic studies suggest that a neoplastic process is most likely [2]. PVNS can affect any large joint, however the knee is most commonly involved and accounts for 80% of cases [1].  PVNS of the knee usually presents insidiously with generalised pain and recurrent swelling, and symptoms are frequently present for many months prior to diagnosis [1].

Lateral patellar instability most frequently occurs in active females in the second decade of life [3]. Over half of cases of initial patellar dislocation are secondary to a sporting injury and MRI is indicated to assess for predisposing morphological factors, including trochlear dysplasia, patella alta and a raised tibial tuberosity – trochlear groove distance, which can be targeted surgically [4].  PVNS is a very rare cause of lateral patellar instability, with a single case previously reported in which nodular disease within the medial patellofemoral joint resulted in mass effect on the patella and lateral subluxation [5].  To the best of our knowledge, we report the first case of localized patellofemoral PVNS subluxing laterally and masquerading as patellar instability, highlighting to radiologists the importance of considering more unusual pathologies as a cause for common symptoms, especially when no other clear cause is identified.

MRI is the imaging modality of choice for the diagnosis of PVNS, which typically presents as a synovial based mass with hypointense foci on T1 and T2 weighted imaging [1].  Iron within the haemosiderin deposited in PVNS causes local magnetic field inhomogeneity, resulting in the characteristic susceptibility (blooming) artefact on gradient echo sequences [6].  Although a rare condition, imaging features are characteristic and help differentiate PVNS from other synovial based pathologies.  Treatment of PVNS consists of total synovectomy, which is curative, however recurrence rates of between 30% and 50% have been reported [2].

Take home message:

• Lateral patellar instability is most commonly mechanical, but rare causes should be considered particularly when no predisposing factors for instability are identified on imaging.
• Blooming artefact on gradient echo sequences helps secure the diagnosis of PVNS and should be considered when there is suspicion on standard MR sequences.
• Interval imaging can be helpful in cases when clinical symptoms of lateral patellar instability progress.

Written informed patient consent for publication has been obtained.