In this study, we found that hip arthroscopy has good clinical outcomes in the treatment of osteoid osteoma of the acetabulum. The pain disappeared immediately after surgery in all patients. The mHHS and iHOT-12 improved significantly one month after surgery and at final follow-up. VAS improved significantly one month after surgery. There was no significant difference in VAS between one month after surgery and final follow-up. The pain disappeared immediately after surgery in all patients.
OO of the acetabulum can be difficult to diagnose. Delay in diagnosis may lead to muscle atrophy, tenderness, localized swelling, possibly contractures, damage of articulation and early osteoarthritis. Previous studies have proved the effect of NSAIDs for treatment of OO and cases of spontaneous healing of OO treated with NSAIDs have been reported.[4, 25] In this study, patients with OO of the acetabulum can also have pain relief by use of NSAIDs. In recent researches, percutaneous resection guided by CT scan, radiofrequency ablation, arthroscopy-assisted radiofrequency ablation and arthroscopic excision for treatment of OO of the acetabulum have been reported.[1, 5, 9-13, 15, 17, 18] In the CT-guided ablation, destruction of the articular cartilage around the lesion is unavoidable and a specimen for pathologic examination may not be able to obtain because of thermal damage.[8, 26] The advantages of arthroscopy are less surgical damage, accurate targeting and excision of the lesion, and treatment of the possible resultant cartilage damage. Synovectomy can be also done during arthroscopic lesion removal, which may prevent cartilage damage, speed the healing process, and relieve pain immediately. We found descriptions of five cases using arthroscopic excision and one case using arthroscopy-assisted radiofrequency ablation for treatment of the OO of the acetabulum.[1, 5, 9, 10, 12, 17] But the number of patients was scare, and there was no clinical follow-up and PROs.
OO of the acetabulum in 2 (33.3%) of all 6 patients were misdiagnosed in another hospital and the 2 patients only underwent femoral osteoplasty and labral repair as their primary surgery. OO of the acetabulum is easy to be misdiagnosed and this feature has been previously described.[6, 12, 27] Two patients in our study underwent revision surgery after radiofrequency ablation guided by CT and arthroscopic excision. Sometimes it is indeed difficult to locate the lesion under arthroscopy. In some patients, cartilage changes could be observed on the surface of the lesion, which could help identifying the lesion. However, sometimes no abnormality was observed in cartilage.
It should be noticed that OO of the acetabulum in 4 (66.7%) patients among all 6 patients was located in zone 5. The other two OO were in zone 4 and zone 6. In the existing studies on arthroscopic treatment of OO of the acetabulum described above, two cases were located in the posterior area,[9, 10] one was in the posteroinferior area, one was at the bottom of the acetabulum, one was in the superior portion of the acetabulum, and one (a 10-year-old boy) was located under the triradiate cartilage. So we could conclude that there is a high incidence of OO in the posterior acetabulum. In our clinical work, we need to focus on this area. Although we used a 70 degrees arthroscope and flexible instruments in the procedure of arthroscopic excision, it was usually difficult and time-spending to get access to the posterior area of acetabulum, especially to zone 5. Excision of OO of the acetabulum in posterior area of hip need suitable equipment, patience and experience.
In this study, 4 (66.7%) patients among all 6 patients had concomitant FAI. We thought that FAI in these patients was secondary to OO. Three (50%) patients had concomitant labral tear caused by secondary FAI. We thought that OO causes repeated inflammatory reaction and bone hyperplasia, which could lead to secondary FAI. Bone hyperplasia of the acetabular fossa and relative lateral movement of femoral head may be a reason of secondary FAI. Further study about secondary FAI is needed in the future.
In addition, one patient in this study had sclerosis of the acetabulum and slight narrowing of the joint space and was diagnosed with Tonnis grade 1 OA. Norman et al. evaluated 30 patients with intraarticular OO of the hip and found OA developed in 50% of those patients. Osteoid osteoma of the hip could stimulate an early onset of osteoarthritis. Repeated inflammatory reactions that damaged cartilage may be the reason of OA caused by OO of the hip. OO of the acetabulum in the joint surface may lead to more direct and severe irritation. Besides, two patients were found to have periosteal reaction in the joint surface of acetabulum, which could be a diagnostic feature of OO of the acetabulum.