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Asymtomatic Extra-Articular Synovial Osteochondromatosis Of The Knee: Report Of A Case

Millar TMS*, Christofides A*, Herlekar D*, Courtman NH*

* Department of orthopaedic surgery, Furness General Hospital, Barrow-in-Furness, Cumbria

Address for Correspondence:

Mr Tim MS Millar
26 Dunkenshaw Crescent
,LA1 4LQ

Tel: 07855320215




The true incidence of extra articular synovial osteochondromatosis is unknown as a proportion of patients will remain asymptomatic. It is thought to be a rare entity with few previous case reports. We report on a case in a 57 year old who despite florid clinical and radiological findings had remained asymptomatic for many years.

J.Orthopaedics 2007;4(3)e20

Case Report:

A 57 year old lumbarjack was admitted to the orthopaedic ward after a heavy log had fallen onto the lateral aspect of his left leg. He presented with pain and swelling over the proximal aspect of the left tibia which was associated with ankle weakness. There were no other further injuries.  

His past history included small vessel vasculitis. His current medication included omeprazole 10mg od, prednisalone 5mg od and azathioprine 50mg od. He also reported a non-progressive swelling located over the lateral aspect of the left knee. This swelling had been present for many years although had not caused him significant symptoms.  

On examination he was found to have no other injuries. Examination of the left leg revealed bruising and tenderness over the proximal fibula. This was association with the long-standing firm smooth nodular swelling over the lateral aspect of the left knee. The swelling was non-tender and measured 7 centimetres by 7 centimetres (Figure 1). The overlying skin was freely mobile with no tethering. He had normal knee range of motion with no effusion and the knee was stable. Neurological assessment revealed grade 1 MRC power in EHL and ankle dorsiflexors. Sensation was reduced in the distribution of the common peroneal nerve.  

Figure 1: Clinical photograph showing mass proximal left tibia and foot drop

Plain radiographs showed no fracture but an incidental finding of multiple calcific nodules which appeared to represent extra-articular synovial chondromatosis (Figures 2 and 3).  

Figure 2: Plain antero-posterior radiograph

Figure 3: Plain lateral radiograph

He was managed with physiotherapy and a foot drop splint. At two week follow up his symptoms had almost completely resolved and he was found to have normal neurological function.


Synovial osteochondromatosis (also called synovial chondromatosis, synovial chondrometaplasia, synovial chondrosis and synovial chondromata) is a benign synovial neoplasm and most commonly is monoarticular. This condition was originally described by Freund 1 in 1937 and is believed to be the result of synovial metaplasia. Further studies suggest that the synovial connective tissue undergoes cartilaginous metaplasia 2 . These cartilage nodules may then be converted to bone through the process of encondral ossification.  

This is a rare condition which may occur intararticularly or extraarticularly. It can occur idiopathically (primary) or as a result of localised joint soft tissue irritation (secondary). It is most often seen in its intra-articular form in the adult population but has also been reported in children 3. Within joints these nodules are thought to form from articular synovial membrane while extraarticular nodules originate from bursal or tendon sheath synovium.  

Osteochondromatosis is thought to be a benign condition but is often frequently recurrent and usually gives symptoms in the form of joint pain, swelling and osteoarthritic changes related to a mass effect 3. Malignant transformation of intraarticular synovial osteochondromatosis can occur but is rarely reported 4,5. Furthermore, to our knowledge, there has been only one report of possible malignant transformation with the extra-articular form 6. If the osteochondromatosis lesion is adjacent to extraarticular bone it is important to rule out other conditions such as synovial sarcoma or periosteal chondroma7.  

The true incidence of this condition is unknown; most of the literature relates to the more common form of intraarticular osteochondromatosis. A thorough medical history and clinical examination should be obtained, followed by plain radiographs and MRI scanning. In this case the patient had been asymptomatic for many years and the radiological findings were incidental. He did not wish further investigation or treatment and had returned to work with normal foot function within ten days of his injury.

Reference :  

  1. Freund E. Chondrosarcomas of the joints. Arch Surg. 1937;34:670
  2. Jaffe HL. Synovial chondromatosis and other articular tumours. In: Tumors and tumorous Conditions of the Bones and Joints. Philadelphia : Lea and Febiger, 1958:566-567
  3. Ko E, Mortimer E, Fraire A. Extraarticular synovial chondromatosis: review of epidemiology, imaging studies, microscopy and pathogenesis, with a report of an additional case in a child. Int J Surg Pathol 12(3):273-280, 2004
  4. Taconis WK, van der Heul RO, Taminiau AM. Synovial chondrosarcoma: Report of a case and review of the literature. Skeletal Radiol 26:682-685, 1997
  5. Blokx WA , Rasing LA, Veth RP, Pruszczynski M. Late malignant transformation of biopsy proven benign synovial chondromatosis: An unexpected pitfall. Histopathology 36:564-566, 2000
  6. Kaiser TE, Ivins JC, Unni KK. Malignant transformation of extra-articular synovial chondromatosis: Report of a case. Skeletal Radiol 5:223-226, 1980
  7. Sim FH, Dahlin DC , Ivins JC. Extra-articular synovial chondromatosis. J Bone Joint Surg Am 59:492-495, 1977


This is a peer reviewed paper 

Please cite as : Millar TMS : Asymtomatic Extra-Articular Synovial Osteochondromatosis Of The Knee: Report Of A Case

J.Orthopaedics 2007;4(3)e20





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