In our study, three (9.1%) patients thought the outcome of ultrasound-guided hip injection was excellent, 8 (24.2%) thought the outcome was good, 9 (27.3%) thought the outcome was fair, and 13 (39.4%) thought the effect of injection was poor. During this period, we performed a total of 750 hip arthroscopic surgeries. Thirty-three (4.4%) in those 750 patients had persistent postoperative pain. The VAS improved from a mean of 5.6 ± 1.4 to 2.0 ± 1.7 10 minutes after injection, changed to 3.3 ± 2.1 at 1 month after injection and improved to 2.4 ± 2.2 at final follow-up. The mHHS, HOS-SSS and HOS-ADL improved from 56.4 ± 10.7, 59.1 ± 8.5 and 44.2 ± 17.1 to 67.3 ± 12.7, 69.1 ± 14.7 and 57.2 ± 23.0, respectively at 1 month after injection and improved to 76.4 ± 11.7, 80.3 ± 14.3 and 69.6 ± 23.0 at final follow-up. There was no significant difference in VAS, mHHS, HOS-ADL and HOS-SSS between " satisfied " group and “unsatisfied” group before injection. Patients in "satisfied" group had older age, higher BMI and more improvement in VAS 10 minutes after injection.
Common causes of postoperative persistent pain included under-resected FAI, residual labral tear, inflammatory reaction, recurrent or misdiagnosed structural pathology.[5, 6] Shapira et al. conducted a systematic review and concluded that the most common indications for revision hip arthroscopy in descending order were labral tears, residual cam deformities, residual pincer deformities, and synovitis. Gao et al. evaluated 21 patients who underwent revision arthroscopy and concluded that misdiagnosed extra-articular impingement, osteoid osteoma and synovial chondromatosis can also be the indication of revision arthroscopy. Mansor et al. indicated that over-resection may also be an important reason of revision arthroscopy and actually cause more profound problems than under-resection. In our study, patients with obvious indication of revision arthroscopy were excluded. Bony development problems like hip instability, femoral anteversion and femoral retroversion were also excluded.
Ultrasound-guided hip joint injection is a kind of safe diagnostic and therapeutic method and can also be a feasible treatment for FAI.[8, 13, 14] Abate et al. evaluated 20 patients with mild FAI and underwent intra-articular ultrasound-guided injection of hyaluronic acid and reported that pain decreased from 6.7 ± 1.3 to 3.7 ± 1.8 and to 1.7 ± 1.8 after 6 and 12 months, respectively and the mean Harris Hip Score improved from 83.3 ± 6 before treatment to 88.2 ± 4.7 at 12 months. Lee et al. evaluated 30 patients with FAI clinically and radiologically and underwent hip injection using steroid or hyaluronic acid and reported that intra-articular hip injection may be effective in FAI, with faster effect of pain improvement by steroid and more delayed effect of function improvement by hyaluronic acid. Zhang et al. reported that ultrasound-guided drug injection can effectively reduce hip pain, improve hip activity, and promote hip functional reconstruction for patients with hip pain after arthroscopic repair of acetabular labral tears. But patients in that study didn’t have diagnosis of FAI. However, there are some studies that reported limited therapeutic effect of intra-articular injection for patients with FAI.[17, 18]
In our study, the mHHS, HOS-ADL and HOS-SSS improved as time went up after injection. There was significant improvement between mHHS before primary surgery and at final follow-up. This proved the effect of surgery and accurate diagnosis. VAS increased one month after injection and improved at the last follow-up. Although VAS increased one month after injection, it was significantly improved compared with that before injection. In this study, there were 22 (66.7%) patients in " unsatisfied " group. The mean VAS before injection and 10 minutes after injection had significant difference in " unsatisfied " group. The significant improvement after injection showed that the cause of persistent pain should be intra-articular pathology. But the mean VAS 10 minutes after injection and 1 month after injection in "unsatisfied" group increased from 2.6±1.7 to 4.4±1.7 and had significant difference (P＜0.05). For patients in "unsatisfied" group, injection didn't provide sustained effect. Although they have temporally relief after injection, the injection didn't solve the problem. Previously published short- and midterm studies have indicated the presence and severity of chondral degradation was predictor of worse clinical outcomes. Chondral degeneration, chronic nonspecific inflammation, muscle strength not recovered, reasons clinical and radiologic evaluation didn't found could be the causes of unrelieved postoperative pain. On the other hand, the mean VAS 10 minutes after injection and VAS 1 month after injection in "satisfied" group had no significant difference (P＞0.05). This proved that the ultrasound-guided hip injection in "satisfied" group has a sustained good effect for treatment of persistent pain after hip arthroscopy. One of the reasons for the improvement is that there may be chronic nonspecific inflammatory blocking recovery. Hip injection could be effective for chronic nonspecific inflammatory. And recovery of muscle strength and function rehabilitation cannot enter the virtuous circle due to chronic pain. The ultrasound-guided hip joint injection may help these patients to enter the virtuous stage of functional recovery for a period of time and improve the clinical outcomes finally. So we think it will help patient to cross the thresholds of recovery and help to delay or avoid revision surgery.
One patient in our study underwent revision arthroscopy 18 months after primary surgery because of unrelieved postoperative pain. Ultrasound examination and MRI before revision surgery didn't identify obvious residual FAI, labral tear or other pathology. However, this patient was diagnosed with labral tear and residual FAI in revision arthroscopy and underwent labral repair, femoral osteoplasty and acetabuloplasty. Inconspicuous labral tear and residual FAI may be misdiagnosed by ultrasound or MRI. Ultrasound-guided hip joint injection can also be a diagnostic tool for persistent postoperative pain. Patients in "unsatisfied" group may also have labral tear or residual FAI that ultrasound and MRI could not identify. For patients with transient relief but recurring pain after hip injection, ineffectiveness of injection in the long term may be one of the indications of revision surgery.
It should be noted that older patients and patients with higher BMI had better treatment satisfaction in this study. The mean BMI of " satisfied " group was 25.1 and the mean BMI of “unsatisfied” group was 22.0 (P＜0.05). Previous studies also reported that obesity may influence the outcome of hip arthroscopy.[20, 21] The mean age of " satisfied " group was 41.1 and the mean age of “unsatisfied” group was 31.9 (P＜0.05). The elderly patients and the patients with high BMI may not able to complete postoperative rehabilitation very well, leading to unsatisfied postoperative results. So ultrasound-guided injection may be more effective in these patients. We also found that the mean VAS 10 minutes after injection in "satisfied" group and "unsatisfied" group was 1±1 and 2.6±1.7, respectively (P＜0.05). This proved that patients who were sensitive to intra-articular injection may have a better long-term clinical outcome. Ultrasound-guided hip joint injection may better help these patients enter the virtuous circle of rehabilitation.
This study has several limitations. First, this study has a relatively small size, because the number of patients with persistent postoperative pain is relatively small. Second, this is not a randomized control study and a control group is lacking. A placebo effect and improvement due to the passage of time cannot be ruled out. However, the persistent good short-term outcomes in "satisfied" group can prove the good effect of ultrasound-guided hip joint injection, not just the effect of passage of time. Another limitation is that HOS-ADL, HOS-SSS and VAS before primary surgery are not recorded.
In conclusion, ultrasound-guided hip joint injection would be a feasible treatment method of persistent pain after hip arthroscopy, especially in older patients, patients with higher BMI and patients who are sensitive to intra-articular injection.