According to statistics, there are about 500,000 people who undergo artificial hip replacement surgery worldwide every year. Among the complications of total hip replacement surgery, heterotopic ossification has been gaining attention in recent years. Heterotopic ossification (heterotopicossification or ectopicossification) refers to the formation of new bone in tissues that do not normally ossify. In addition to occurring after total hip replacement, it is also commonly seen in patients with central nervous system trauma, burns, and joint fracture dislocation. The incidence of heterotopic ossification after total hip arthroplasty has been reported differently, ranging from a low of 0.6% to a high of 90%. The earliest foci of heterotopic ossification can be seen on radiographs 2 to 3 weeks after surgery and mature after 1 year. Mature ectopic bone is very similar to normal bone in terms of histological and imaging manifestations and is sometimes misdiagnosed as parosteal osteosarcoma or synovial sarcoma on the basis of its radiographs. Brooker et al. classified these foci into five grades according to their severity: Grade 0: no ectopic osteosarcoma formation on radiographs; Grade I: multiple isolated islands of bone in the soft tissues surrounding the hip joint; Grade II: bone spur growth in the pelvis and/or proximal femur, the distance between the two is greater than 25px; Grade III: distance between bone spurs is less than 25px; Grade IV: bony ankylosis of the hip joint. They concluded that this classification correlates well with the Harris score and has a high clinical utility. This classification has been adopted by a large number of authors.
The etiology of heterotopic ossification is unclear and is generally considered to be related to a variety of factors. Heterotopic ossification is more common in men, especially in patients with ankylosing spondylitis, diffuse idiopathicspinalhyperstosis (DISH) and severe hyperplastic arthritis in the postoperative period, while it is very rare in patients with congenital hip dislocation.Fahrer et al. reported that patients with preoperative DISH The incidence of postoperative heterotopic ossification was 29%, compared to 10% in controls. However, there are still some clinical studies that fail to confirm this relationship. The more severe the hyperplasia, the higher the incidence of postoperative heterotopic ossification.
Sodemann et al. reported 56 cases of total hip replacements on both sides, and all patients had bilateral heterotopic ossification of the hip joint. A number of authors have also reported a significantly increased risk of heterotopic ossification in one hip on the other side. Others have reported a higher risk of postoperative heterotopic ossification in those with higher preoperative serum alkaline phosphatase levels, but recent studies have failed to confirm this. It has also been suggested that heterotopic ossification is associated with blood sedimentation.
In addition to systemic factors, heterotopic ossification after artificial total hip arthroplasty may also be affected by the following local factors.
1, preoperative factors: some people found that patients with a history of hip surgery are prone to heterotopic ossification after artificial hip arthroplasty, and the risk is higher for those who have already had ossification after previous surgery. Brooker et al. reported that in 100 cases of total hip replacement, the incidence of postoperative heterotopic ossification was 21%, while the rate of postoperative heterotopic ossification in 14 cases with a history of ipsilateral hip surgery was 50%, which was significantly higher than that of those without a history of hip surgery.
2, intraoperative factors: almost all factors related to surgical operation are suspected to have the potential to cause heterotopic ossification. It is thought that the surgical approach may play a very important role. However, the different approaches, mainly the lateral and anterior incisions, are still of mixed opinions, and a significant number of case reports fail to confirm the relationship between heterotopic ossification and the surgical approach. Errico et al. reported that in 100 total hip replacements, the incidence of heterotopic ossification was 22% in the group with large ramus osteotomy and 13% in the group without osteotomy, suggesting that large ramus osteotomy should be avoided in total hip arthroplasty.
(1) congenital dysplasia of the hip joint.
(2) Abnormal acetabular morphology.
(3) Severe flexion and external rotation contracture of the hip joint.
(4) Difficult revision surgery. However, some people do not agree with the above-mentioned view.
Tissue damage caused by surgery is also an important factor that is suspected. Prolonged surgery, muscle strain, incomplete hemostasis, and residual inactivated muscle and bone debris have all been suggested as possible causes of heterotopic ossification. It has also been suggested that, in particular, metal cuproplasty requires revision of both the femoral head and the acetabulum, thus increasing the chance of residual bone fragments and thus increasing the risk of heterotopic ossification.
The occurrence of heterotopic ossification does not seem to be related to the type and material of the prosthesis. Whether it is the more used Charnley prosthesis and metal cup, or the popular Bateman bipolar prosthesis in recent years, postoperative heterotopic ossification has a high incidence, including cobalt-chromium, titanium, ceramic and other materials are not immune. It was suspected that the application of bone cement was related to heterotopic ossification, but in recent years, it was found that heterotopic ossification also occurs after total hip replacement without bone cement.
3. Postoperative factors: The postoperative complications related to heterotopic ossification reported so far include hematoma, infection, dislocation and prosthesis loosening. Anticoagulant drugs are often used to prevent deep vein thrombosis after total hip replacement, but statistics show that the incidence of heterotopic ossification after heparin anticoagulation is higher than that of other anticoagulant drugs such as hydroxyprogesterone and aspirin.
Prevention
Many authors believe that careful and gentle intraoperative manipulation, thorough irrigation and debridement, proper use of perioperative antibiotics and anticoagulants, and postoperative closed drainage can help reduce the incidence of heterotopic ossification after total hip replacement. In addition, the following measures have been taken in recent years for prevention in high-risk patients.
1, diphosphonates (diphosphonates or bisphosphonates) drugs: their physicochemical properties are similar to pyrophosphate, and contain two C-P bonds. Brunner et al. applied HEBP at a dose of 20 mg/kg/day from 1 month before to 3 months after surgery, and no heterotopic ossification was observed after surgery. However, many authors have observed that heterotopic ossification can occur soon after discontinuation of the drug, and therefore concluded that HEBP only has a delaying effect on heterotopic ossification but does not prevent its occurrence. In addition, long-term application of diazoxide can cause bone softening, so these drugs have been used less often.
2, radiation therapy: In 1981, Coventry and Scanlon first reported local radiotherapy for total hip replacement patients with an irradiation dose of 20Gy/10 times/12 days, and serious heterotopic ossification was found at the end, so it is believed that radiotherapy can prevent heterotopic ossification in the early postoperative period. 92%. Subsequently, the irradiation dose was reduced to 10 Gy and found to be as effective as the 20 Gy dose. Lo et al. changed the treatment regimen to a single irradiation of 7 Gy within 72 hours postoperatively and achieved a success rate of 96%, with only one case of grade II ossification and no clinical symptoms. although the follow-up period was only 6 months, the preventive effect was still encouraging. Koski et al. compared 10 Gy/5 times with 8 Gy/time The results showed no difference in efficacy between the two regimens and suggested that the indications for radiotherapy prophylaxis are ankylosing spondylitis, proliferative arthritis, DISH and patients with a history of hip surgery and heterotopic ossification. Although no effect of radiotherapy on wound healing has been observed, a number of studies have demonstrated that radiotherapy at any dose can cause nonunion of the greater trochanteric osteotomy. In addition, radiotherapy can affect the degree of biological fixation after total hip replacement with porous surfaces. In view of this, some people limit the irradiation field during radiotherapy to the femoral head neck and its medial and lateral areas, while the acetabulum and femoral stalk facing the prosthesis are not irradiated.
3. Nonsteroidal anti-inflammatory drugs (NSAIDs) have the main effect of inhibiting prostaglandin synthesis and thus non-specifically suppressing the inflammatory response. Other authors have reported the ability of these drugs to inhibit the wandering and differentiation of mesenchymal cells. Ritter and Sieber reported a 10% incidence of grade I and II heterotopic ossification compared to 2% for grade III in high-risk patients who had total hip replacement with the use of indomethacin as prophylaxis. In a randomized, double-blind prospective study by Schmidt et al. In contrast, in the control group, heterotopic ossification occurred in 72 of 89 cases, of which 48 cases had grade III ossification. Gastrointestinal reactions and even central nervous system symptoms sometimes occurred after taking NSAIDs. In recent years, the use of aspirin, eubufen, diclofenac and other drugs has been reported, and the side effects of some of these drugs have been reported to be significantly less than those of anti-inflammatory pain. It has also been reported that the growth of bone into the micro-pores on the surface of the prosthesis is inhibited after the use of drugs, resulting in a decrease in the strength of the interface between the prosthesis and the bone.
Treatment
Most heterotopic ossification after total hip replacement does not require treatment, and treatment is limited to those with significant hip pain or functional impairment. Although surgical excision of postoperative heterotopic ossification has been attempted since the 1970s, a significant number of authors have concluded that it is unnecessary and that the recurrence rate after surgery is quite high. For example, Fahmy and Wroblewski resected the heterotopic ossification foci at the same time as the revision of the artificial hip, and the recurrence rate was 92%. Some authors have improved the method of removing the heterotopic ossification foci alone; Abrahamson removed the heterotopic ossification foci and then free grafted adipose tissue around the femoral neck of the prosthesis, and the hip was pain-free at 1 year of follow-up, with an average increase of 6O° in hip flexion range and no severe heterotopic ossification. MacLennan et al. performed resection of ectopic ossification in 53 hips with severe ectopic ossification and postoperative radiotherapy (20 Gy/10 times/5 days), and only one case of recurrence was observed.
Regarding the timing of resection, there is a wide divergence of opinion. It is generally believed that surgery should be performed only after the ossification foci have matured, and early resection is often ineffective. The osteoclastic shadow can be seen 2 to 3 weeks after surgery on X-ray, but a clearer image can be seen only 3 months after surgery. It has been suggested that the size and distribution of the ossification foci will not change after 6 months, but others believe that the formation of heterotopic ossification is a dynamic process and may change in size or distribution within 1 to 5 years, so it is difficult to determine the maturity of heterotopic ossification foci based on radiographs. The former is not only highly sensitive but also quantitative, while the latter can show the relationship between the ectopic ossification foci and the surrounding muscles, blood vessels, and nerves, which can guide the surgical resection. Although the preliminary reports are relatively satisfactory today, further systematic studies are needed. The current majority opinion favors that maturation of heterotopic ossification foci generally takes 1 year.