Multiple small-diameter drilling was performed by one of us (Zhenfeng Yuan) between April 2011 and June 2012 in 39 patients (64 hips) at the same institution for the treatment of non-traumatic osteonecrosis of the femoral head. Of these patients, 21 (34 hips) received simvastatin after the procedure. Eighteen patients (30 hips) who refused subsequent treatment with simvastatin were taken as controls. Patients in the MD group underwent core decompression with multiple small-diameter drilling. Patients in the SIM group underwent multiple drilling combined with oral simvastatin, 20 mg per day for 12 months beginning on the day after the drilling procedure. In the MD group, two patients (four hips) were lost to follow-up; therefore, 16 patients (26 hips) were available for study. In the SIM group, one patient (two hips) was lost to follow-up; therefore, 20 patients (32 hips) were available for study (Fig. 1). Observations were truncated 36 months postoperatively to permit statistical comparison between the two groups. This study was approved by the ethics committee of Liaocheng People’s Hospital and informed consent was obtained from all individual participants included in the study.

Osteonecrosis of the femoral head was diagnosed if magnetic resonance imaging (MRI) (coronal and sagittal) revealed the following: belt-shaped or circular low intensity signals surrounded by high intensity signals in the outer area on short tau inversion; or a high intensity area surrounded by belt-shaped or circular low intensity signals within the femoral head on T1-weighted images [24]. The patients were classified according to the ARCO system [25]. Patients were eligible for inclusion in the study if they were older than 18 years, had hip pain and had ARCO stage I, II or IIIA ANFH. Patients were excluded from the study if they had ARCO stage IIIB, IIIC or IV ANFH. The procedure was not performed on patients older than 60 years.

The largest anteroposterior diameter of the head (R), the longest anteroposterior length of the necrotic lesion (A) and the longest mediolateral length of the necrotic lesion (B) were measured on axial slices on MRI. The extent of the necrotic portion was calculated by the equation: percentage of necrotic lesion = {(A × B)/R²} × 100. This method has a more acceptable accuracy and is reasonably repeatable compared with other methods [26]. All hips were assessed using this method, with necrotic portions greater than 30 % considered large lesions, 15–30 % medium lesions and less than 15 % small lesions. The location of the necrotic lesion was classified as A, B, C1 or C2 according to the classification developed by the Specific Disease Investigation Committee under the auspices of the Japanese Ministry of Health, Labor and Welfare [27]. Three experienced radiologists reviewed all MRI scans and plain radiographs together in a blinded fashion.

Patients were followed-up 1, 3, 6, 9, 12, 24 and 36 months after the procedure. In addition, they received a follow-up visit whenever they were unwell. Follow-up took the form of outpatient visits. Examinations included joint pain, function and range of motion. HHS and VAS pain score were recorded. Plain radiographs (anteroposterior and frog lateral view) and MRI scans of both hips were obtained at scheduled follow-up times. The primary outcomes of the study were clinical and radiological failure. Clinical failure was defined as HHS < 75 points or a requirement for subsequent hip surgery such as bone grafting, osteotomy or hip replacement. New occurrence of collapse or increased collapse of greater than 2 mm on plain radiographs during follow-up was defined as radiological failure [28, 29].

For the drilling procedure, patients were placed in the supine position on a fracture table. A C-arm fluoroscope was draped with a sterile sleeve and positioned over the hip region to enable an anteroposterior view. After the position of the femoral head had been marked and the hip draped, a 3.0 mm Steinman pin was inserted percutaneously under fluoroscopic guidance. The pin was inserted into the femoral head at the site of the lesion. Anteroposterior and lateral fluoroscopic views were necessary while advancing the pin to ensure that it remained in the medullary canal of the femoral neck. Depending on lesion size, each femoral head was drilled three to six times. The wound was closed using a simple bandage without suture.

Patients who had undergone unilateral drilling were advised to use two crutches when walking for 6 weeks after the procedure. Two crutches when walking for 12 weeks were advised for patients who had undergone a bilateral procedure. Patients were allowed to engage in physical activities and sports 12 months after surgery. No further rehabilitation program was implemented [28].

Statistical analysis of the data was performed using SPSS version 18.0 for Windows (IBM, Armonk, NY, USA). Baseline characteristics were presented using descriptive statistics. The chi-square test was used to compare nominal data. The t-test or Mann–Whitney U test was used to compare metric data. Univariate analyses were performed using the chi-square test. Variables with a P value of less than 0.10 on univariate analysis were entered into multivariate analysis. Causes of ANFH were also entered into multivariate analysis. Multivariate analysis was performed using a logistic proportional hazards regression model. Because of the multicollinearity between ARCO stage and lesion size, models were fitted using the forward conditional procedure. All statistical assessments were two-sided and evaluated at the 0.05 level of statistical significance.

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