There are few reports of posterior knee HO in the literature. In a study by Garland et al.  three cases of HO of the knee were reported. In only one of them the lesion was located in the posterior knee and none of the cases developed ankylosis of the knee. In the series published by Charnley et al.  and Ippolito et al.  no cases of popliteal space HO were reported. To our knowledge, there are rare reports of excision of large popliteal space HO. In a report by Anderson and Lais  a large HO mass was excised from popliteal fossa of a 20 years old man 17 weeks after traumatic brain injury. The patient had a fixed flexion contracture of 45 degrees. At 7 months follow-up the patient had a range of motion of 10-125 degrees. They used both irradiation therapy and indomethacin for postoperative prophylaxis. We found no reports of popliteal space HO except the aforementioned study. Our patient is unique in that his knees were fixed in full extension and that surgical intervention for resection of the lesion was done 5 years after development of the HO. We attribute the postoperative residual flexion deficit at least partly to the contracture of extensor mechanism in extension during the long period of time.
Spinal cord injury is a well known predisposing factor for development of HO. The incidence of HO after spinal cord injury has been reported to be 20-25% . The most common joints involved are hip, shoulder, elbow and the knee in order of decreasing frequency. Involvement of knee joint with HO has marked effect on functional status of the patients significantly reducing their adaptive capacity for daily living [6, 7, 9]. Fuller et al.  reviewed 17 patients with 22 knees involved by heterotopic ossification and categorized their sitting impairment and investigated their functional outcome after resection of the lesions. He classified the patients as: group I (patients who are able to use a wheelchair or a chair without being assisted), group II (patients who can use chair only with the help of assistive devices such as cushions or chair extensions) and group III (patients who are not able to use chair even with assistance).
Multiple researchers have shown the benefit of surgical excision of HO lesions of the knee in overall functional status of the patients [6, 9, 10]. Traditionally, the optimal time for resection of heterotopic ossification was considered to be after maturation of the lesion (normalization of bone scan). This was thought to reduce the recurrence of the lesion. Recently, earlier surgical intervention has been recommended by some authors. Melamed et al. reported excision of 12 HO lesions in 9 patients . Despite increased uptake on bone scans in all patients, recurrence did not occur in any of them. They suggested that increased uptake on bone scans is not a contraindication to surgical excision of HO, provided the neurologic status is stabilized. Importance of neurological status of the patients and its impact on the results of surgery has been emphasized by other authors. Sarafis et al.  attributed the poor functional outcome of their patients after excision of HO in 22 hips to their uncontrolled neurologic syndrome. They recommended accurate evaluation of the preoperative neurologic status. On the other hand, they warned about the risk of fracture in delayed surgery due to localized osteoporosis. Delay in surgical intervention may also have a detrimental effect on regaining the range of joint motion, adversely influencing the efficacy of rehabilitation programs.
The exact etiology and pathophysiology of HO is not clearly defined. Chalmers et al. studied the inducing capacity of different tissues for bone formation . He believed the presence of three conditions is necessary for development of ossification within soft tissues: 1) an inducing agent; 2) an osteogenic precursor cell; and 3) an environment which is permissive to osteogenesis. A large amount of information regarding the pathophysiology of HO has been collected by studying the cases of myositis ossificans progressiva; an inherited disorder with progressive debilitating ossification of soft tissue structures [18–23]. The role of bone morphogenic proteins (BMPs) and its antagonists such as noggin has recently been the focus of attention. It is postulated that the BMP-4 gene itself may not be defective but a defect in the genes that code BMP-4 antagonists leads to suppression of inhibitory mechanisms and overexpression of BMP-4 .
Recurrence of HO after surgical resection is one of the most common complications affecting the final outcome. The role of prostaglandine E2 (PGE2) in pathophysiology of HO and its increased urinary excretion in early stages of the disease has been the rational for use of non steroidal anti-inflammatory drugs (NSAIDs) as a preventive measure. Indomethacin has been of particular interest. Indomethacin appears to be effective in the primary prevention of HO after spinal cord injuries and after total hip arthroplasty and as secondary preventive measure after resection of HO lesions . The major drawback of indomethacin use is the increased risk of operative bleeding, its gastrointestinal side effects and its negative effect in bone union. Other more selective NSAIDs have been studied for this reason and their efficacy and safety is under investigation. Radiation therapy has been used extensively for the prevention of HO. Many side effects seen with the use of indomethacin are not the concern with irradiation. With proper shielding, irradiation can be applied to only where it is needed. However, despite the low doses used for HO prophylaxis, the risk of carcinogenesis is a concern. Most articles about the effects of radiation therapy in prevention of HO focus in post-total hip arthroplasty (THA) cases. The studies about the preventive effects of radiation therapy are plagued with small sample sizes and inadequate research protocol design. The optimal dose and fractionation of dosage are subjects of some researches .
Popliteal space HO is a rare affliction. With presentation of our case, we believe that by resection of popliteal space knee HO, good function and improvement of life style can be anticipated even after a long delay in presentation. Appropriate postoperative prophylaxis with radiotherapy and NSAIDs should be considered in treatment course.