Isolated lateral meniscus tear masquerading as a potential septic arthritis in a 7 year old

Abstract

A 7-year-old boy presented with a 1-week history of a limp, low grade temperature and mildly elevated inflammatory markers. He sustained a fall 2 weeks ago but was relatively symptom free for a week. The inital physical exam demonstrated a knee effusion clinically, radiographs did not demonstrate any overt pathology and biochemical testing demonstrated an elevated C reactive protein of 8 mg/L. An arthroscopy was conducted, with a plan for synovial biopsies to investigate for infective versus inflammatory arthritis as a cause for the effusion. An isolated lateral meniscal tear was discovered. This was repaired using all inside technique with Smith and Nephew’s FastFix 360 (R) suture anchors. It is quite uncommon to find isolated lateral meniscus tears in this population, but the learning point demonstrates it is important to arthroscopically examine the whole knee, even while performing synovial biopsies as there may be unexpected findings.

Background

Paediatric meniscal injuries are rare, usually occuring in conjunction with either a ligamentous or bony injury.^{1 2} Isolated meniscal injuries are very uncommon, with only small numbers being reported throughout the literature.³ The incidence of meniscal injuries in those younger than 9 years is thought to be 1 per 100 000 patients, and these often occur in conjunction with anterior cruciate or tibial avulsion injuries.³ There are small numbers of case reports of isolated meniscus injuries occuring; however, these are more often medial meniscal injuries.^{4 5} There is just one case report of an isolated lateral meniscus tear occuring in a discoid meniscus in a 6-year-old girl.⁶ A large retrospective cohort review of paediatric and adolescent patients in a specialist centre revealed that patients who sustain lateral meniscus tears, usually sustain a posterior horn (71%) followed by a peripheral, then a vertical tear.² There were 50% of the lateral meniscal tears associated with anterior cruciate ligament injuries.² The preferred approach for the paediatric population is repair if possible. Meniscectomy is less desirable, due to the risk of arthritis in later life.⁷

Case presentation

We present the case of a 7-year-old boy who presented with a 1-week history of limp, lateral knee pain and a low grade temperature of 37.8°C. The pain was described as a 5 out of 10, not fully relieved with oral analgesia. It was laterally and anteriorly based. Despite these symptoms, the patient could still weight bear and flex/extend the knee. He had a history of a minor injury 2 weeks previously, whereby he was running around in circles, while holding a mobile phone and slipped in the kitchen. However, the family describe him returning to normal behaviour without apparent pain after 1 hour. The symptoms of knee pain and swelling developed 1 week later. He had a trial of simple analgesia, rest, elevation and ice at home; however, he disimproved and presented to the emergency department (ED) for review. His vital signs in the ED were normal. On examination, he had a knee effusion, with minor warmth, no cellulitis, bruising, abrasions or erythema was evident. His range of movement was 5° of extension to 130° of flexion. His hip range of movement was normal. He had no other systemic findings. The Kocher criteria are a set of diagnostic criteria for septic arthritis of the hip; however, they are not validated for septic arthritis of the knee.

Investigations

A radiograph was performed in the ED, which did not reveal any bony injury. The radiograph initially was reported as a potential lucency in the distal femoral metaphysis, potentially representing early osteomyelitis. Biochemical investigations in the form of a full blood count, renal profile and C reactive protein (CRP) were performed. The CRP was elevated at 8 mg/L (normal range in local hospital of 0–<5 mg/L). The rest of the investigations were non contributory.

Differential diagnosis

The differential diagnosis included either an infectious or inflammatory aetiology. The history of a low grade temperature, with an effusion on examination and a questionable radiograph prompted the decision to undertake a diagnostic arthroscopy with a view to joint aspiration and synovial biopsy. This would enable the surgeons to be able to perform culture, microscopy, histopathology and PCR on any samples to narrow down the diagnosis. The reason for proceeding to arthroscopic synovial biopsy was that the radiograph had demonstrated a potential lucency. It was decided by the consultant that a joint aspirate alone would not provide enough information in this setting to make a definitive diagnosis. A decision was made not to obtain a magnetic resonance image preoperatively due to the same concerns as well.

Treatment

The patient underwent a knee diagnostic arthroscopy. This was carried out under aseptic technique, with a thigh tourniquet. Antibiotics were administered according to local guidelines after a fluid sample and synovial biopsy was taken. A routine arthroscopy of the whole knee was conducted and the lateral meniscus tear was discovered (figure 1). This was probed and found to be acute, with a bucket handle type orientation. The posterior and anterior horns were intact. The medial compartment, cruciates and cartilagenous surfaces were found to be normal. The edges of this isolated lateral meniscus tear were debrided to a stable rim, and repaired using the Smith and Nephew’s FastFix 360 (R) suture anchor system (figure 2). This was performed using an all inside technique, with attention to visualise the popliteus during the process. This repair was probed and found to be stable.

Figure 1.

Arthroscopic image of lateral meniscus complex tear, with stable anterior and posterior horn.

Figure 2.

Repair of complex lateral meniscus tear with FastFix (R) suture anchor.

Outcome and follow-up

The patient was managed initially in a hinged knee brace, with partial weight bearing, increasing range of movement and weight bearing status over a period of 6 weeks. The cultures and synovial biopsy were negative for infection. The aspirate revealed scanty red blood cells only. The pain resolved and the patient returned to baseline. The final follow-up at 12 months demonstrated the patient was pain free, with a 0/10 visual analogue pain score. The range of movement had returned to normal, with full flexion and extension.

Discussion

The function of the meniscus is to act as a shock absorber, to aid in joint lubrication, proprioception and nutrition.⁷ These injuries are common in adults but rare in the paediatric population. Most cases present with locking or restricted range of movement, and are associated with other injuries like tibial spine avulsion fractures, structural abnormalities like discoid meniscus or cruciate injuries.⁸ A meta-analysis of 373 patients with isolated meniscal injuries found the youngest reported patients varied from 12.2 years with a mean age of 14 years.⁸ This review demonstrated superior outcomes for repair versus meniscectomy in the paediatric population. Medium-to-long-term follow-up for repair has sustained benefits and may avert the development of post-traumatic arthritis, in a susceptible population.⁹ The earlier literature demonstrates poorer outcomes with partial meniscectomy, with patients at an increased risk of developing arthritis.^{7 8} Outcome scores after repair report success rates up to 90% with respect to self-reported outcome measures, with a failure rate of up to 17.3% in the literature.¹⁰ This case demonstrates the importance of performing a diagnostic arthroscopy, in conjunction with an arthroscopic washout.

The usual approach to a suspected meniscal tear in an older patient would involve obtaining MRI. This was not performed in this case, as the differential diagnosis included inflection or inflammation. It was not felt that MRI would have helped to sufficiently clarify the situation.

It is important to have the available technology to repair these meniscal injuries, as the reported outcomes are superior for repair, versus conservative therapy. Other options for repair include inside-out techniques. It is accepted that the paediatric population have better outcomes with repair than meniscectomy.⁸

Learning points.

• Meniscal tears can occur in a young paediatric population.

• The differential for knee effusions with a recent history of trauma can include infection, inflammatory arthritis or intra-articular fractures/ligamentous or meniscal pathology.

• A diagnostic arthroscopy for the young paediatric populations should specifically seek to exclude ligamenotus injuries, chondral injuries and meniscal pathology. It is worth putting the arthroscopy camera into the knee, rather than just ‘washing it out’ without visualisation.

• Be prepared to repair these injuries, with appropriate anchors on standby if a diagnostic arthrosopcy is being undertaken.