Is there added risk in resurfacing a femoral head with cysts?
© Gross and Liu; licensee BioMed Central Ltd. 2011
Received: 16 February 2011
Accepted: 17 October 2011
Published: 17 October 2011
Femoral head cysts have been identified as a risk factor for early femoral failures after metal-on-metal hip resurfacing arthroplasty (HRA) based on limited scientific data. However, we routinely performed HRA if less than 1/3 of the femoral head appeared destroyed by cysts on the preoperative radiograph. This study was undertaken to analyze whether there was an added risk of early femoral failures in HRA when femoral head cysts were present.
This retrospective case-control study included 939 MOM HRAs operated by a single surgeon with use of the posterior minimally invasive surgical (MIS) approach between November 2005 and January 2009. Patients with all diagnoses except osteonecrosis were included. Among them, 117 HRAs had femoral head cysts ≥ 1 cm identified in surgery. All cysts were treated with bone grafting using acetabular reamings packed into the cavitary defect (instead of filling the cysts with cement). The control group, which had no cyst observed at the time of surgery, was randomly selected from our database using computer algorithms to match those cases in the study group for the parameters of surgical date, age, gender, body mass index, diagnosis, femoral fixation method, and the size of the femoral component.
The minimum follow-up was 24 months for both groups. The early femoral failure rate in the study group was 3/117 (2.6%) and 0/117 in the control group; there was no statistical difference between these two groups (P = 0.08). In the study group, there were two femoral neck fractures (revised): both occurred in patients having a cyst size of 1 cm3; and there was one femoral component loosening at 3-year follow up in a patient having a cyst size of 2 cm3.
Although the risk of early femoral failures among the group with cysts appeared higher than the group without cysts, we could not demonstrate a significant statistical difference between the two groups. It is possible that bone grafting cysts rather than cementing them may account for the low failure rate, and that this technique may minimize the risk of resurfacing a femoral head with cysts.
Hip resurfacing arthroplasty (HRA) with metal-on-metal bearings has become an established and viable hip arthroplasty option for the younger patient with higher activity levels due to bone preservation. This technique may also make revision surgery less complicated [1, 2]. In Europe, the rate of resurfacing has varied between 6% and 9% with 6% in France, 9% in Germany, and 7% in the UK [1, 3]. In Australia, the hip resurfacing accounts for 7.9% of all hip arthroplasty procedures. In some countries, hip resurfacing has been utilized in up to 50% of all hip arthroplasties in patients younger than 55 with a low revision rate of 2.8% at five-year follow-up post-operatively [4, 5].
The risk factors for stemmed total hip arthroplasty (THA) appear to be different than for HRA5, and many experts have advocated that HRA may be more advisable in certain subsets of patients with severe degenerative arthritis of the hip. Risk factors have been proposed that increase the risk for HRA [6–8]. Femoral head cysts are widely believed to increase the chances of early femoral failure in HRA; however, the only scientific data that exist now to support this idea is mainly from Beaule's study [6, 9, 10]. In their study, femoral head cysts were identified as a risk factor for early femoral failure after metal-on-metal HRA as a part of the proposed Surface Arthroplasty Risk Index (SARI) . Cysts were found to be a significant risk factor (P = 0.028) for early femoral failure. Our concern is that the technique of managing cysts may be important in achieving a good outcome. In Beaule's study, cysts were filled with cement; our technique is to instead fill them with acetabular reamings prior to cementation or uncemented fixation.
We were not convinced that cysts affected the failure rate provided that they involved less than one third of the prepared femoral head and that they were bone grafted instead of being filled with cement. Because the scientific evidence to support cysts as an independent risk factor was limited, we have routinely used this approach. After many years of experience with these cases, we have now undertaken this study to independently analyze what the added risk of early femoral failure in HRA was when femoral head cysts were present and treated with bone grafting. Our hypothesis in this retrospective case-control study was that femoral heads with cysts involving less than 1/3 of the prepared femoral head did not significantly affect the early femoral failure rate after HRA.
Demographic and diagnosis comparison between the groups with or without cysts.
-- with Cyst
-- Without Cyst
11/2005 to 1/2009
8/2005 to 12/2008
Number of hips
Number of patients
Age at surgery (years)
53 ± 6 (range: 35 to 69)
53 ± 5 (range: 34 to 65)
189 ± 40 (range: 110 to 290)
186 ± 37 (range: 110 to 275)
Body mass index
27 ± 4 (range: 19 to 39)
27 ± 4 (range: 20 to 39)
T-score (Bone mineral density)*
0 ± 1 (range: -2.5 to 3.3)
0 ± 1 (range: -2.4 to 3.5)
The information of the cyst size among the study group.
Size of Cyst (cm3)
Summary of the Surgical Information between the groups with or without cysts.
-- with Cyst
-- Without Cyst
2 ± 1 (range: 1 to 3)
2 ± 1 (range: 1 to 3)
Hospital stay (days)
2 ± 1 (range: 1 to 5)
2 ± 1 (range: 1 to 7)
Operation time (min)
120 ± 23 (range: 85 to 242)
109 ± 17 (range: 80 to 168)
Size of cyst (cm 3 )
1.8 ± 0.8 (range: 1 to 4)
Femoral component size (mm)
51 ± 4 (range: 44 to 62)
51 ± 4 (range: 44 to 60)
Fixation of femoral component
Fully porous coated (Uncemented)
The level of significance was set as 0.05 (α = 0.05) for all comparison tests in this study. The paired t tests were performed to compare the numeric variables between pre-operative and post-operative visits. The standard t tests were performed to compare the differences between numeric variables of the study and control groups. Chi-square tests were performed to evaluate the difference of categorical variables between these two groups. The Kaplan-Meier curves were used to analyze the survivorship rates using revision of femoral components as the end point among these two groups. The Chi-square tests were performed to approximate the results of the Wilcoxon tests in order to compare the differences of survivorship functions between groups. The null hypotheses of all of these tests were that the survivorship functions were the same between the two compared groups . Also, the Pearson Chi-square tests were utilized to compare the differences of failure rates between groups without considering the time variable.
Summary of clinical outcomes between the groups with or without cysts.
-- with Cyst
-- Without Cyst
Period of follow-up (months)
42 ± 11 (range: 24 to 61)
45 ± 12 (range: 24 to 65)
54 ± 12 (range: 24 to 91)
55 ± 13 (range: 21 to 83)
97 ± 6 (range: 68 to 100)
95 ± 8 (range: 71 to 100)
8 ± 2 (range: 4 to 10)
8 ± 2 (range: 3 to 10)
VAS score in the regular day
0 ± 1 (range: 0 to 4)
0 ± 1 (range: 0 to 4)
VAS score in the worst day
1 ± 2 (range: 0 to 8)
1 ± 2 (range: 0 to 7)
Number of femoral failures (revisions)
Detailed information of early femoral component failures in the group with cysts.
Time after surgery (Months)
Cyst size (cm3)
Femoral size (mm)
Reason of failure
Treatment of failure
Femoral Neck Fracture
Femoral Neck Fracture
Femoral Component Loosening
Excluding the revised cases, the average post-operative HHS scores at the latest follow-up visit was 97 ± 6 in the study group and 95 ± 8 in the control group; both were improved significantly from the average pre-operative HHS scores, respectively (P < 0.001) (Table 4). There were no significant differences in the UCLA activity and VAS pain scores on the regular or worst days. Radiological analysis revealed that no hip showed evidence of femoral radiolucency or migration.
When comparing HRA to stemmed THA, the spectrum of complications is different. Considering that multiple bearing options are currently available for stemmed THA, the comparison between HRA and stemmed THA becomes even more difficult. Two complications that are unique to HRA are femoral neck fractures and postoperative femoral head osteonecrosis. We have therefore decided to focus on these. In combination, they represent early femoral component failures after HRA. Proximal femoral bone preservation in young active patients is the primary reason that metal-on-metal HRA was developed. However, if the risks of early femoral failures are particularly high in a certain group of patients, they may be considered poor candidates for HRA. If the alternative risks of amputating the femoral head and neck to perform a stemmed THA are much lower in this group, the theoretical advantage of bone preservation with HRA in younger patients may no longer be worthwhile. Numerous studies have focused on delineating risk factors for HSR to help the surgeon decide which patients may have too high a risk with HRA to make proximal femoral bone preservation worthwhile [6, 10, 14, 15]. Unfortunately, it is not always clear exactly why a certain risk factor is problematic. Does a smaller component size lead to more problems because of a small area of femoral fixation5? Or is the problem with smaller components primarily because of more adverse wear problems [10, 16, 17]. The present retrospective case-control study was specifically undertaken to assess one proposed risk factor for early femoral failures: Does the presence of femoral head cysts increase the risk of early femoral failure?
Cysts in the femoral head are areas where bone loss has occurred due to the arthritic process. Therefore, it is generally believed by experts that femoral head cysts negatively impact the success rate of HRA [6, 9, 10]. However, to our knowledge, only few papers have reported scientific evidence that femoral head cysts are a risk factor for HRA 5. Because it seems logical that cysts might affect femoral fixation, this belief has largely gone unchallenged, despite the fact that the evidence available is limited. Beaule et al 5 proposed a SARI on the basis of a study of 92 HRAs done in patients under 40 years of age. The average follow-up was 3 years (range: 2-5.6 years). Survivorship with revision for early femoral failure as an endpoint was 97% (two femoral neck fractures, one femoral loosening). There were two additional radiographically loose femoral components (migration) and eight additional possibly loose femoral components (complete stem radiolucency). This formed the problematic group (N = 13). A univariate analysis of multiple risk factors was done. Points were assigned to certain risk factors based on their odds ratio in this analysis. Two points were assigned for cysts > 1 cm3, 2 points for weight under 82 kg, one point for UCLA Activity score above 6, and one point for previous hip surgery. The maximum score was 6. The SARI was found to be significantly higher in the 13 problematic hips than in the remainder of the hips in the series (P < 0.001). Femoral head cysts were found in 53% of well-functioning hips while they were present in 92% of problematic hips (P = 0.028). Their data implicate the presence of femoral head cysts (>1 cm3) as a risk factor for HRA. It does not quantify the added risk for failure due to cysts. Also, the cysts in Beaule's study were managed by debridement and filling with cement.
Comparison of the results between Beaule & Amstutz's study and the present study.
Beaule & Amstutz
Gross & Liu
# of patient
# with cysts > 1 cm 3
# without cysts
Follow-up length (yrs)
3 (range: 2 to 5.6)
3.5 yr (range: 2 to 5.4)
UCLA activity score
Femoral revision rate
Femoral radiological loosening
P value of femoral component failures between cyst and non-cyst group
In summary, our study, with a control group matched for other previously proposed risk factors for early femoral loosening, could not demonstrate that femoral head cysts were an independent negative risk factor for failure of the femoral resurfacing component. However, we caution that this may be due to the way we treat femoral head cysts with bone grafting, rather than filling them with cement. We therefore recommend that the presence of cysts within the femoral head, as long as they comprise less than 1/3 of the remaining prepared femoral head, be eliminated as a risk factor for HRA. We suggest that other surgeons consider bone grafting cysts rather than filling them with cement. Comparison studies to further compare these two techniques would be valuable.
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