Audible knee snapping is a rare occurrence and with few cases reported. Gout is an inflammatory arthritis; and in chronic gout, cartilage degeneration and scar tissue formation occurs.

Gout usually affects the peripheral joints such as the big toe, ankle, and wrist [1]. Gouty arthropathy of the elbow and knee joints is not uncommon [2]. Gout in the shoulder or hip joints is uncommon [3].

We report a case of knee gout, presenting as persistent audible knee snapping. We also discuss audible noises that arise with knee range of motion and their proposed etiologies.

A 26-year-old Chinese male with no significant previous medical history presented with three months of audible right knee snapping. He works in information technology and plays recreational football weekly. There was no recent knee trauma, knee pain, or instability. The knee snapping occurs when he ranges his knee, from extension to flexion. He feels the patella catching on the lateral femoral condyle associated a loud, painless snap.

There was no knee effusion and his lower limb alignment was normal. The collateral and cruciate ligament examination was normal. His right knee range of motion was full with an audible knee snapping that occurs when he ranges his right knee from extension to flexion (Video 1).

The plain film knee radiographs revealed a mild lateral patella tilt (Figure 1, Figure 2, Figure 3). The magnetic resonance imaging (MRI) of the right knee revealed mild anterior cruciate ligament (ACL) degeneration, degenerate lateral meniscus posterior horn root, and chondromalacia (Figure 4, Figure 5, Figure 6).

He was started on physiotherapy with quadriceps and gluteal strengthening. After six months, there was no improvement in his symptoms, instead the snapping became more frequent and louder. He had to stop all sports as the snapping became increasingly frequent.

The impression was patella malalignment with audible snapping. He was offered arthroscopic surgery for intraoperative assessment of the right knee patella tracking. He underwent a right knee arthroscopy, chondroplasty, scar tissue release, and lateral retinacular release. The menisci were noted to be normal. There were extensive uric acid deposits seen in all compartments of the right knee intraoperatively (Figure 7 and Figure 8). A lateral retinacular release was performed to help improve patella tracking (Figure 9). Intraoperatively, the patella was not catching the lateral femoral condyle with knee range of motion after the lateral release (Video 2).

Postoperatively, the audible knee snapping resolved with improved patella tracking (Video 3). The histology confirmed the multiple deposits of chronic tophaceous gout. Subsequent blood tests revealed that he had elevated uric acid levels of over 600 mmol/L (normal value < 450 mmol/L). The patient was informed of the diagnosis of gout and he was referred to by a rheumatologist for management of hyperuricemia. The patient remains well one year after the surgery.

Audible noise that comes with knee range of motion is a definite source of patient concern. This happens uncommonly with few cases reported in the literature. These audible knee noises are described as: snapping, cracking, squeaking, and clunks.

Knee snapping is described as a patient hearing or feeling a snapping or popping of the joint while performing a specific activity [4]. It can be associated with an intra-articular lesion like a discoid meniscus [5]. As the patient’s knee flexes, it can be confirmed on arthroscopy that the posterior horn of the discoid lateral meniscus moves posteriorly and the central portion of the discoid lateral meniscus moves anteriorly simultaneously, creating a snap at deep flexion. The central portion is returned to the original position accompanied by snapping at near full extension [6].

Mariani et al. reported that arthroscopic examination revealed that inflammation and fibrosis of the popliteus tendon can cause audible snapping in the knee. Dynamic arthroscopic examination of the popliteus tendon was especially useful when observing the popliteus tendon subluxate or dislocate from the popliteal sulcus at 30° to 40° as the knee passed from flexion to extension or vice versa [7].

Knee snapping can also be caused by extra-articular structures, the hamstring tendons and the iliotibial band (ITB) [8],[9],[10],[11]. Medial knee snapping can be caused by the hamstring tendons subluxation over the posteromedial corner of the tibia [8],[9] or the hamstring tendons moving over a medial proximal tibia osteochondroma [10].

Snapping pes tendons have been reported after total knee arthroplasty (TKA): due to residual proximal tibia bony prominence, a change in alignment after TKA or a tight ITB. The tight ITB with postoperative scarring results in abnormal patella tracking, generating an audible snap with knee range of motion. This was managed with ITB release by multiple small punctures [11],[12]. This is similar to our case because the knee snapping was a result of patella maltracking.

Two theories have been proposed regarding knee cracking sounds: the sudden collapse of a cavitation bubble [13],[14] and the formation of a clear space or bubble [15],[16]. Kawchuk et al. presented evidence from real-time MRI to show that knee cracking is related to cavity formation. This is consistent with tribonucleation, a process where opposing surfaces resist separation until a critical point where they separate quickly, creating a sustained gas cavity [17].

Squeaking, occurs with edge loading of hard surfaces after the loss of fluid film lubrication, which may be caused by impingement or third body particles [18]. Squeaking has been classically a potential complication after ceramic-on-ceramic bearing total hip arthroplasty, with an incidence of 0.7–20% [19].

There have been two postoperative cases of knee squeaking reported after medial patellofemoral ligament (MPFL) reconstruction [20]. The authors believe that the braided, nonabsorbable suture caught in the joint space resulted in an audible squeak. The authors also suggested that patella maltracking with friction can cause audible knee squeaking.

In addition, some patients are aware of a grinding sensation or popping coming from their knee after knee replacement [21],[22]. This phenomenon was labeled as patellar clunk in case studies that described the knee catch or clunk patients experienced after undergoing posterior cruciate substituting TKA [21].

The lesson that we can draw from our case is that audible knee snapping likely originates from the patellofemoral joint, due to patella maltracking. For this patient, superior patella and lateral retinacular scarring contributed to the maltracking. In addition, the deposits of uric acid on the cartilage resulted in secondary osteoarthritis, with increased surface friction. This aggravated his abnormal patella tracking. The lateral release and scar removal have helped to improve the patella tracking and resolved the audible knee snapping post-surgery.

Gout has also been associated with osteoarthritis and joint damage [23]. Histological studies have reported that monosodium urate (MSU) crystals are deposited within or adjacent to cartilage lesions [24]. Joint damage is a late feature of the disease and includes bone erosion, shown to be closely associated with tophus deposition [25]. The extensive uric acid deposits on the cartilage were not reported on the MRI. Even retrospectively when the MRIs were re-evaluated, these deposits were also not clearly seen. The second important lesson is the fact that the MRI may not be able to see fine gout deposits on the cartilage.

Davies et al. stated that tophi demonstrate variable imaging appearances on MRI, reflecting their heterogeneous components and the diagnostic performance of MRI in gout remains uncertain [26]. This highlights the limitation of the MRI in diagnosing uric acid deposits.

The three stages in the natural history of gout are: gout flares, intercritical gout, and chronic gouty arthritis. The stages of gout usually present sequentially; clinical severity of disease parallels the frequency of gout flares and the eventual development of chronic gouty arthropathy. However, there are reports of patients with chronic gouty arthritis or tophaceous gout, in the absence of a prior gout flare [27].

This is similar to our case as prior to presentation, he had no previous history of gout flares and was previously not diagnosed or treated for gout. However, there were extensive gout deposits in the knee consistent with chronic gouty arthritis. It is therefore important to remember of gout as a “great mimicker.” It is often mistaken for joint infection, cellulitis, and even osteomyelitis. This is the third learning point from this case. Gout can present in a multitude of ways and should remain a diagnosis that we should always consider for joint pathology.

In our case, gout arthropathy has resulted in extensive cartilage MSU deposits, lateral retinacular and suprapatellar scarring, leading to patella maltracking and audible knee snapping. This is the first case seen in literature for audible knee snapping caused by chronic gout arthritis with patella maltracking.

Gout can present in a multitude of ways and should be considered as a possible diagnosis in joint pathology. Magnetic resonance imaging may not be sensitive enough to detect fine uric acid deposits in the knee joint. There can be extensive uric acid deposition in the knee joint, resulting in joint degeneration and patella maltracking. This can present as audible knee snapping; and can be managed with arthroscopy, debridement, and lateral release.

The patient and family were informed that data from the case would be submitted for publication, and gave their consent.