8.9 Miscellaneous conditions around the knee

The causes of knee pain are most often related to overuse or trauma. Other conditions can mimic common knee disorders and should be considered in the differential diagnosis in patients who present with a history and physical findings inconsistent with mechanical knee pain.

Synovial chondromatosis is a monoarticular disease presenting more often in males, in the third and fourth decades of life. Clinically, patients present with knee effusions and episodes of locking and grinding. Radiographically, in the early stages of this disease, an effusion is present, but as the disease progresses, characteristic ‘popcorn’ calcifications are found in and around the joint (Figure 8.9.1).

Synovial chondromatosis is a metaplastic disease process whereby synovium modulates into cartilage. Hyaline cartilage coalesces on the fronds of synovium and becomes intra-articular loose bodies. When these cartilaginous loose bodies mineralize, this is called synovial osteochondromatosis.

If unrecognized, cartilage damage can occur from repetitive mechanical trauma resulting in joint arthrosis. Treatment involves either synovectomy (arthroscopic or open) and rarely in recalcitrant disease, chemical or radiation synovectomy. If intra-articular osteocartilaginous loose bodies are present, their removal in addition to synovectomy is necessary. Recurrences after synovectomy are relatively uncommon.

Rarely, malignant transformation occurs, usually marked by underlying osseous destruction and intra-articular nodular masses. Early diagnosis is important and either extra-articular resection or amputation is necessary to achieve local tumour control. Histologically, loss of the clustering growth pattern, myxoid stromal changes, necrosis, and spindling of cells in the periphery of the lobules are suggestive of malignant change. Pulmonary metastases may occur following malignant transformation.

Pigmented villonodular synovitis is a rare benign disorder affecting synovial joints and tendon sheaths. The knee is the most commonly involved large joint. It has been regarded as a reactive process, but recent cytogenetic evidence suggests a neoplastic aetiology. Two types of pigmented villonodular synovitis exist—a localized form is characterized by a solitary lesion, whilst the diffuse form is more aggressive in nature and usually involves the entire synovial membrane.

Local disease is usually heralded by mechanical symptoms such as locking and catching. Diffuse disease is characterized by pain, swelling, stiffness, and eventually deformity. Diffuse pigmented villonodular synovitis can invade local structures extensively, including muscle, tendon, and bone, with periarticular cysts, erosions, and narrowing of the joint space. Magnetic resonance imaging (MRI) is useful in determining the nature of the disease (Figure 8.9.2). Multinodular intra-articular disease is characterized by patchy areas of fat and haemosiderin specific for pigmented villonodular synovitis.

With local disease, treatment is frequently successful with marginal complete arthroscopic or local excision, but with diffuse disease recurrence is greater than 45%. Arthroscopy and open total synovectomy alone may not relieve symptoms in these patients. With active diffuse disease confined to synovium, total synovectomy can give good short-term results. However, where the disease has extended out of the joint and into bone, local disease control is difficult. Arthrodesis and total joint replacement may be necessary for patients with diffuse and destructive disease.

Radiotherapy using intra-articular instillations of fibrosing agents is effective for refractory cases. External-beam radiotherapy has been used successfully as a salvage procedure for residual diffuse disease.

Even in patients with active disease, total synovectomy with total knee replacement will give good long-term results with recurrence of disease unlikely.

Soft tissue ganglia arise from capsular, aponeurotic, bursal, and ligamentous structures as a result of myxoid degeneration of collagen fibres.

Extra-articular ganglia are usually painless or minimally symptomatic soft tissue masses located behind the knee in the popliteal fossa and originating from the posterior capsule. These masses do not communicate with the joint and insinuate around the semimembranosus tendon and the medial head of the gastrocnemius muscle. Popliteal or Baker’s cysts are reported incidentally in 10–41% of MRI examinations of the knee. These masses can be found in children but are more commonly seen in adults, and the incidence increases with patient age. They are usually diagnosed incidentally and more commonly in adults with underlying mild to moderate arthritic changes in one or more compartments of the knee.

Synovial cysts can also arise in the tibiofibular joint and present as a painful effusion associated with restricted knee flexion and potentially compressive and invasive neuropathies of the common peroneal nerve.

These encapsulated fluid collections can also arise within the knee joint, most commonly the tibial insertion of the anterior cruciate ligament and can cause mechanical pain, restriction of motion, and locking of the knee.

Meniscal cysts arise from degenerative horizontal tears of the menisci. Lateral cysts are three to four times more common than medial cysts; however, medial cysts tend to be larger.

The best diagnostic study for evaluating cystic and cyst-like structures about the knee is MRI. These lesions appear uniformly hyperintense on T₂-weighted sequences and hypointense on corresponding T₁ images. Meniscal tears are also well visualized on MRI. It is important to establish a connection to the joint or aponeurotic structure because other lesions like myxoid sarcomas or synoviosarcomas can occur in the popliteal fossa and be mistaken for a common ganglion. Neurilemmomas arising from the nerve sheaths passing through the popliteal fossa can also mimic ganglia. However, these lesions are typically painful and are associated with dysaesthesias when compressed or percussed. On MRI they have an elongated appearance attenuated above and below (comet sign) and are in continuity with a nerve, often exhibiting a target sign (Figure 8.9.3). Moreover, on T₂-weighted sequences, they appear to be more heterogeneous with alternating bright and dark signals corresponding to intermixed cellular areas (Antoni A) and hypocellular areas (Antoni B) with superimposed haemorrhage and haemosiderin deposition in more long-standing lesions (ancient schwannomas).

Giant cell tumour of the tendon sheath is an occasional extra-articular mass occurring about the knee. Typically patients are in the fourth to sixth decade and present with pain and swelling that is mostly activity related. Giant cell tumour of the tendon sheath is a nodular form of pigmented villonodular synovitis and arises in tendons and aponeurotic structures. It is also heterogeneous on T₂-weighted sequences with varying degrees of dark signal depending on the degree of haemosiderin and collagen deposition within the tumour. Histologically, there are monomorphic round to spindled stromal cells and interspersed giant cells within a collagen matrix with moderate vascularity and haemosiderin-laden macrophages. The aetiology of this tumour is unknown. Local recurrences are uncommon after marginal or even intralesional excision.

These fluid-filled voids are often adjacent to cruciate or meniscofemoral insertions. Most are asymptomatic but can, on occasion, be quite extensive and painful due to subchondral insufficiency.

Radiographs show a small well-defined radiolucent abnormality that is subchondral with sclerotic borders. MRI may show a fluid-filled abnormality that is bright on T₂ with little oedema in the adjacent dome. Subchondral cysts are reported in 1% of routine MRIs of the knee and treatment is not necessary.

Congenital and developmental anomalies may result in morphological changes that clinically and radiographically can be confused with neoplasia.

Dwarfing conditions, such as achondroplasia, thanatophoric dysplasia, and spondyloepiphyseal dysplasia, because of abnormal physeal maturation, can cause peculiar changes in epiphyseal or metaphyseal development that may be mistaken as a tumour. Other conditions, such as chondroectodermal dysplasia or Ellis–von Crevald syndrome are associated with proximal diaphyseal exostoses. This is an autorecessive condition characterized by short stature, polydactyly, wide but dysplastic lateral tibial plateaux, and hypoplastic nails.

Chondrodysplasia punctata or Conradi–Hunermann syndrome is an autosomal dominant condition marked by asymmetric shortening with punctate mineralization of the epiphyses among other characteristic clinical features.

Metaphyseal destruction can be observed in patients with congenital syphilis, rubella, cytomegalovirus, and toxoplasmosis. Metastatic neuroblastoma and Wilms’ tumour can also present as knee pain with corresponding destructive radiographic changes that require further clinical and radiographic staging followed by biopsy to diagnose and treat appropriately.

Multiple hereditary exostoses is also an autosomal dominant condition caused by a defect in the ossification groove of Ranvier or perichondral ring resulting in horizontal physeal growth and multiple periarticular osteocartilaginous excrescences that can be painful. These patients are of low to normal stature with abnormalities of the paired bones associated with valgus deformities of the elbows, wrists, knees, and ankles. Pain may result from tendonitis, bursitis, fracture through the exostosis, osteonecrosis of the cartilaginous cap, and malignant transformation which can occur in up to 5% of affected individuals. Malignant transformation is rare before skeletal maturity and is usually marked by increased growth of the mass associated with pain that is present with activity and rest. Other rare complications of osteochondral exostosis are compressive neuropathies, particularly around the knee, and vascular claudication, especially large lesions in the popliteal fossa.

Radiographically, osteochondral exostoses are metaphyseal or metadiaphyseal based at the ends of long bones. They typically grow away from the physis pointing away from the joint. On computed tomography (CT), a diagnostic feature is corticomedullary continuity between the tumour stalk and underlying medullary canal. The cartilage cap is usually thin, with a thickness of less than 1.5 cm; however, in a growing child and in proximal appendicular and axial sites, it can exceed 1.5cm and raise suspicion for malignant transformation. In patients with malignant change, apart from an excessively enlarged cap, there is underlying destruction of bone and mineralization within the cap appearing as arcs and rings, stipples, or flocculations. Dystrophic calcification can be seen in tumours undergoing osteonecrosis, and the clinical appearance of acute pain along with these radiographicchanges which evolve over several weeks or months can arouse suspicion for malignant transformation. Ultrasound scanning and MRI are useful in screening for malignant changes. Osteochondral exostoses can be prodigious in the proximal humerus and proximal femur, but most commonly occur as solitary masses about the knee.

Treatment is observation for asymptomatic lesions. For patients with pain, excision through the base of the stock to include the cartilage cap and overlying bursa is necessary. Axial imaging is important for preoperative assessment of the relationship of the mass to adjacent neurovascular structures. If malignant transformation is suspected, staging studies are necessary prior to biopsy and definitive surgical management.

This is a rare condition characterized by epiphyseal-based osteochondral proliferation of tissue that causes pain, joint incongruity, and early arthrosis if untreated. This condition may involve more than one joint.

Radiographically, there is intra-articular osteochondral tissue growth resulting in joint asymmetry and incongruity. Fluid sensitive Short Tau Inversion Recovery (STIR) MR images show hyperintense signal in the periphery reflecting the high fluid content in articular cartilage. Treatment involves resection and contouring of the joint surface to create a congruous joint. Early arthrosis is likely despite treatment and certain without.

Enchondromas are ‘cartilaginous rests’ arising from physeal cartilage (Box 8.9.3). The cells do not undergo scheduled apoptosis and calcification, but instead become entrapped in bone and migrate away from the physis with continued growth (Figure 8.9.4). The size varies from a few centimetres to involvement of nearly the entire shaft of a long bone. Most enchondromas are asymptomatic and recognized incidentally in the evaluation of more common traumatic and degenerative knee conditions. However, they may be biologically active following skeletal maturation and cause pain and discomfort, and thus are considered ‘active enchondromas’.

Rarely, a solitary lesion can undergo malignant transformation over time, marked clinically by increased pain, occasional pathological fracture, and a soft tissue mass. Radiographically, deep endosteal scallops, greater than two-thirds of the cortex, cortical destruction, and the presence of a soft tissue mass strongly suggest malignant transformation.

Patients with multiple enchondromas (Ollier’s disease) or multiple enchondromas associated with haemangiomas (Maffuci’s disease) are at increased risk of developing chondrosarcoma; this risk is reported to be 25% in affected individuals.

Osteogenesis imperfecta, a disease of abnormal bone collagen, is manifest by frequent fractures and bowing deformities. In the tarda form of disease the clinical manifestations, and hence diagnosis, may not be as apparent, raising the possibility of child abuse in some children. Occasionally, fractures about the knee, particu-larly the distal femur, may heal with exuberant callus associated with massive swelling and be confused with an aggressive sarcoma. This diagnosis is particularly difficult for the unsuspecting pathologist, since large atypical osteoblasts with hyperchromatic nuclei coupled with mitotic activity arouses suspicion of osteosarcoma.

This is a common variant found in adolescent patients at or near the insertion of the medial gastrocnemius muscle in the posterior medial aspect of the distal femur. Almost all are found incidentally. This abnormality is commonly mistaken for an aggressive neoplasm, specifically osteosarcoma. Radiographically, it is a cortically-based radiolucent abnormality without matrix production but with an internal sclerotic border. The well-delineated border is best seen on CT, while MRI shows a small soft tissue mass and surrounding oedema. They can be quite large and are often bilateral. Observation is the preferred treatment.

This common condition occurs in young active adolescents, presenting with activity-related anterior knee pain and mild aching discomfort when sitting for periods of time. It is most common in males and is a traction apophysitis with occasional fragmentation and hypertrophic bone formation at the insertion of the patellar ligament.

Pain can be elicited by resisted quadriceps contraction and direct pressure over the tibial tubercle. Treatment consists of rest, activity modification, anti-inflammatory medicines, and isometric quadriceps exercises. Steroid injections should be avoided to prevent later patellar ligament rupture.

Avulsion injuries of the medial collateral ligament with subsequent repair may result in mineralization at or near the femoral insertion of the ligament, known as Pellegrini–Stieda disease. It is asymptomatic and found incidentally on trauma radiographs for other conditions. A remote history of injury is usually elicited, while physical examination may yield normal to mild valgus opening on stressing the knee. It should not be confused with a periarticular soft tissue mass such as synovial sarcoma or a periosteal osteosarcoma. The mineralization pattern is smooth regular and consistently located at the insertion of the medial collateral ligament in the distal femur.

This is a common condition often confused with fracture and primary tumours. Unlike fracture, the edges of the bipartite patellar fragments are smooth, non-tender to palpation, and located in the superolateral aspect of the patella. This condition is best appreciated on oblique knee radiographs. There is no joint effusion unless there is an unrelated intra-articular injury or other knee pathology. In most cases, the lesion is bilateral.

Blunt and, less commonly, penetrating injuries of the soft tissue can result in haematoma, inflammation, and necrosis of muscle and fat, which, in the process of repair, can modulate into bone formation. Patients relate a history of fairly significant trauma, most often a deep contusion. Over several weeks to months, this indurated area becomes a firm, sometimes fixed, mass which becomes progressively less painful. Restricted joint motion is common immediately after an injury but improves with time. Late pain and discomfort is usually secondary to mechanical irritation, whereas night symptoms and rest pain are not present. Generalized soft tissue myositis with joint contractures and ankylosis can be seen in patients with acute pancreatitis and burns. Fibrodysplasia ossificans progressiva can present with generalized soft tissue periarticular masses as well as joint ankylosis. This disease is transmitted in an autosomal dominant fashion, though most cases are sporadic. It is generally manifested in early childhood as a large, rapidly progressive soft tissue mass that is often confused with sarcoma. The diagnosis can be made by inspecting the hands and feet, which reveal shortened first rays. In the foot, a common feature is brachymesodactyly with a portion of the proximal phalanx fused to the head of the first metatarsal giving the appearance of a hallux valgus deformity. It is important to recognize these constellations of physical and radiographical findings to avoid biopsy or attempted resection of the soft tissue mass which may worsen the condition.

Other radiographical findings include exostoses, hypoplastic posterior spine elements, a shortened anteroposterior diameter of the cervical spine, and a shortened femoral neck. Radiographical mineralization is observed in the axial and appendicular skeleton. Knee contractures and ankylosed joints occur later in the disease. Patients generally succumb to pneumonia and respiratory failure due to involvement of the intercostal muscles and marked spine and chest wall deformity. The heart and diaphragm are spared.

Isolated myositis around the knee must be distinguished from other mineralized soft tissue masses such as surface-osteosarcoma. Radiographically and histologically, myositis is characterized by a ‘zonation phenomenon’ whereby maturation progresses from the epicentre of the lesion to the periphery. For malignant tumours, maturation proceeds from peripheral to central, thus the periphery of the tumour is least differentiated. The mineralization pattern in soft tissue sarcomas is disorganized and the radiographical pattern is ‘dystrophic’.

Like parosteal osteosarcoma, myositis ossificans can be separated or densely adherent to bone giving it a ‘pasted on’ appearance. Parosteal osteosarcoma invades the cortex of bone in over two-thirds of cases and involves the medullary canal in one-third. Both parosteal osteosarcoma and myositis ossificans can be differentiated from osteochondral exostoses by the absence of corticomedullary continuity.

Osteochondritis dissecans lesions are relatively common in young patients and adolescents. The typical manifestations of the disease are knee pain and an effusion; occasionally mechanical symptoms occur in patients with loose bodies or detached osteochondral fragments. The aetiology remains unclear. The majority of lesions are in the weight-bearing portion of the lateral aspect of the medial femoral condyle. Lateral femoral condylar lesions are less common and felt to be post-traumatic in nature resulting from patella subluxations and dislocations. Tibial osteochondral defects are rare.

These lesions are typically seen on radiographs as subchondral lucencies with internal sclerotic borders; occasional loose osteochondral bodies can be seen in the lateral or medial gutters or the suprapatellar pouch. Both CT and MRI are useful in discerning the exact size and location of the chondral defect. The appearance and location are characteristic and should not be confused with chondroblastoma, subchondral cysts, or Brodie abscess. Treatment depends on the size and degree of displacement of the osteochondral fragment.

Spontaneous osteonecrosis of the knee (SONK) most commonly involves the medial femoral condyle and medial tibial plateau and is generally idiopathic. Secondary osteonecrosis is associated with systemic conditions such as rheumatoid disease, Caisson’s disease, alcohol consumption, renal transplantation, steroid therapy, and systemic lupus erythematosus.

SONK is characterized by the presence of dead bone in the subchondral area of the weight-bearing portion of the femoral condyle. It is associated with subchondral fracture and collapse.

The aetiology remains uncertain but local trauma in a diseased joint causes ischaemia and disturbance of local circulation leading to increased subchondral pressure and ultimately necrosis.

Radiographs may appear normal in the early stages of osteonecrosis but with progression of the disease an area of radiolucency in the subchondral bone with a halo of sclerotic bone becomes evident (Figure 8.9.5). MRI detects early changes in the bone marrow, whilst technetium bone scans are less reliable. MRI findings include a well-defined area of low signal, best seen on T₁-weighted images. Patients with condylar lesions greater than 5cm² or a width of greater than 40% develop pain, deformity, and secondary destruction of the joint often requiring surgical intervention.

Non-operative treatment in the early stages of the disease includes pain relief and supportive physiotherapy exercises to improve quadriceps and hamstring strength, resulting in full resolution of symptoms in 80% of cases. Effective surgical options include arthroscopic debridement, drilling and bone grafting, and proximal tibial osteotomy. However, total or unicompartmental knee arthroplasty is indicated for lesions evolving into subchondral collapse and significant arthrosis. Success rates are 55% for arthroscopic procedures and up to 95% for arthroplasty (Figure 8.9.6).

A rare periosteal or cortically based hyperostosis involving the long bones of the lower extremity. It can be very extensive or more limited about the knee. In those lesions where it is more focal it can be confused with other bone-producing conditions such as juxtacortical osteosarcoma or myositis ossificans.

Radiographically, melorheostosis involves only one side of the cortex and only one lower extremity. It often crosses joints, especially the knee. It has been described as having the appearance of ‘dripping candle wax’.

Pain is a constant feature of this disease. Often patients will present with a mass and knee effusion. With disease progression, ankylosis can supervene, and pain due to progressive hyperostosis may rarely require amputation for palliation.