Abstract
Background:
Periprosthetic fractures have long been recognized as one of the major complications of hip resurfacing arthroplasty. The objective of this study was to develop a systematic and morphologic classification of the fracture mode based on pathogenesis.
Methods:
One hundred and seven retrieved specimens consisting of the femoral remnant and the femoral component of a total hip resurfacing arthroplasty that had failed as a result of a periprosthetic fracture were analyzed with regard to the morphologic failure mode. The location of the fracture line was used to differentiate the fractures. The fractures were also classified histopathologically as acute biomechanical, acute postnecrotic, or chronic biomechanical.
Results:
Fifty-nine percent (sixty-three) of the fractures occurred within the bone inside the femoral component. Fifty-one percent (fifty-five) of the fractures were classified morphologically as acute postnecrotic; 40% (forty-three), as chronic biomechanical; and 8% (nine), as acute biomechanical. Acute biomechanical fractures were found exclusively in the femoral neck and occurred earlier (mean time [and standard deviation] between implantation and revision, 41 ± 57 days) than acute postnecrotic fractures (mean time between implantation and revision, 149 ± 168 days; p = 0.002) or chronic biomechanical fractures (mean time between implantation and revision, 179 ± 165 days; p = 0.001). The latter two fracture types both occurred predominantly in the bone inside the femoral component.
Conclusions:
Three distinct fracture modes were characterized morphologically. Osteonecrosis was the most frequent cause of fracture-related failures. We suggest that an intraoperative mechanical injury of the femoral neck such as notching and/or malpositioning of the femoral component might lead to changes in the loading pattern or in the resistance to fracture of the femoral neck and may result in both acute and chronic biomechanical femoral neck fractures. These findings may serve as feedback information for the surgeon and possibly influence future therapeutic strategies.
With the introduction of wear-resistant materials (specifically, metal-on-metal bearings), total hip resurfacing arthroplasty is emerging as an alternative to conventional total hip arthroplasty as a prosthetic solution for young patients with end-stage osteoarthritis of the hip1. It has been suggested that this type of arthroplasty has several advantages, such as more precise biomechanical restoration2, decreased morbidity at the time of revision arthroplasty3, normal femoral loading and reduced stress-shielding4, reduced dislocation rates5, and a decreased prevalence of thromboembolic phenomena as a benefit of not using instruments in the femur6.
Recent studies have established a 96% prosthetic survival rate at five years7 and a 95% rate at seven years8. Femoral neck fractures, which have a prevalence of 1.0% to 3.0%9-15, and loosening, which has a prevalence of 1.0% to 2.0%9,12, have been identified as the main causes of failure11. Knowledge about the pathogenesis of femoral head and neck fractures is still limited. Studies of risk factors have highlighted the importance of patient selection and bone quality12 in addition to errors in surgical techniques or use of inappropriate indications13 for implant survival.
The primary goal of this study was to identify different modes of fracture in specimens retrieved during revisions of failed total hip resurfacing procedures. Using macroscopic examination, contact radiographs, and histological evaluation, we analyzed characteristics of fracture-related failures in order to explain the mechanisms leading to this short-to-medium-term complication.
Patient Cohort
From January 2004 to December 2007, a series of 152 femoral head remnants with the femoral component in situ that had been retrieved during revision total hip resurfacing arthroplasties were analyzed with macroscopic and histopathological examination and contact radiographs. One hundred and seven hips were revised because of a periprosthetic fracture, and they constitute the study cohort. Of the remaining forty-five hips, eleven were revised because of failure of the acetabular component and thirty-four were revised because of loosening or groin pain.
The original diagnosis was known for eighty-one patients, sixty-two (77%) of whom had been treated for advanced stages of osteoarthritis. The other pathological conditions were trauma (six patients, 7%), hip dysplasia (six patients, 7%), rheumatoid arthritis (four patients, 5%), and osteonecrosis of the femoral head (three patients, 4%). Fifty-seven patients were men (mean age [and standard deviation], 57 ± 8 years), forty-three were women (mean age, 55 ± 11 years), and the sex of the remaining patients was unknown. One hip of each patient was analyzed in the present study; there were no bilateral revisions. Valid data on the duration of implantation were obtained for ninety-eight individuals, and only these cases were included in the analysis of the time between implantation and revision.
Materials
The specimens were obtained from patients who had been included in an international multicenter study16,17. All specimens were fixed in buffered formalin immediately after the revision surgery. Five different devices (ASR [articular surface replacement], DePuy Orthopaedics, Warsaw, Indiana; Durom, Zimmer Orthopaedics, Warsaw, Indiana; Cormet, Corin Medical, Cirencester, United Kingdom; ReCap, Biomet Orthopaedics, Warsaw, Indiana; and BHR, Smith and Nephew, London, United Kingdom) were included in the study.
Methods
After macroscopic photographic documentation of the specimens, one central 4-mm slice of the femoral head remnant with the femoral component in place was cut in the coronal plane with use of a diamond-coated saw (EXAKT, Norderstedt, Germany). The implant-bone composite as well as the cement interface was left intact. A contact radiograph of the slice was made (Faxitron X-Ray LLC, Lincolnshire, Illinois), the slice was photographed macroscopically, and the pictures were stored digitally. After dehydration and fat removal, the undecalcified specimens were infiltrated by exposing them to ethanol-methylmethacrylate solutions with increasing concentrations of methylmethacrylate (Technovit 7200; EXAKT/Kulzer, Norderstedt, Germany); the infiltration was completed with use of methylmethacrylate combined with benzoyl peroxide under vacuum conditions for fourteen days in order to guarantee complete preservation of the cement18. The specimens were then ground with an automatic grinding device (EXAKT), processed to a thickness of 300 µm, and stained with toluidine blue dye. The thickness of the slices was chosen to enable two and three-dimensional analyses18. One additional slice was cut perpendicular to the original coronal plane, and the slices were completely embedded in plastic for conventional histological analysis17. Three quadrants were analyzed morphologically. From each undecalcified processed plastic block, three 5-µm sections were cut with a heavy-duty microtome (Carl Zeiss, Oberkochen, Germany). The sections were stained with von Kossa, toluidine blue, and Goldner trichrome stains.
Macroscopic Analysis
All specimens were evaluated on the basis of the location of the fracture line with respect to the distal border of the femoral component. The specimen was considered to have an inside (femoral head) fracture when the fracture line was completely or partly in the bone inside the femoral component, and it was considered to have an outside (femoral neck) fracture when the fracture line was completely outside of the bone encompassed by the resurfacing cup. Osteonecrosis was characterized by the finding of yellowish-to-creamy-white areas. The degree of bone-marrow perfusion with blood was also documented in order to identify osteonecrosis. Linear intraosseous white-bluish cartilaginous areas were considered to represent pseudarthrosis.
Analysis of Contact Radiographs
Irregular linear zones of sclerosis suggested formation of callus or new bone within the fibrotic zone distal to the area of osteonecrosis. Osteolytic changes within the zone of sclerosis or at its border were suggestive of pseudarthrosis.
Histopathological Analysis
Osteonecrosis was defined histologically as bone trabeculae without stainable osteocytes and with disorganized bone marrow. Bordering fibrosis with few scattered lipophagic histiocytes, appositional reparative new bone formation, and hyperemic blood vessels were detected at the distal border of areas of osteonecrosis in the majority of cases. Linear callus formation or pseudarthrosis was considered to be a steady, chronic reaction of living bone. Fragmented bone trabeculae (bone chips) and the absence of reparative changes characterized a recently occurring event.
Bone-marrow edema syndrome was considered to be the diagnosis in specimens with depletion of hematopoiesis, diffuse or spotty accumulations of interstitial and intrasinusoidal fluid, scattered lipophagic histiocytes, and loose fibrosis between the fat cells19-22.
Statistical Analyses
The type-I error probability was set at 5%. Since the time between the implantation and the revision surgery clearly deviated from a normal distribution, nonparametric analytical methods (the Kruskal-Wallis test and the chi-square test) were applied. Weighted kappa analysis was used to calculate the reliability of intraobserver agreement (with one of the authors [J.Z.] performing a second observation after nine months) and the reliability of independent interobserver agreement (between two of the authors [J.Z. and M.A.]) for both the macroscopic and the histopathological diagnoses. Kappa values were interpreted according to an often-used scale23. The standard deviation is used throughout to describe the spread around the mean.
Source of Funding
The study was financially supported by Biomet Orthopaedics (Warsaw, Indiana), Corin Medical (Cirencester, United Kingdom), DePuy Orthopaedics International (Leeds, United Kingdom), Smith and Nephew (London, United Kingdom), and Zimmer (Warsaw, Indiana).
The mean time of the revisions of the 107 arthroplasty failures due to fracture was approximately five months (152 ± 164 days) after implantation. In eleven cases, pseudarthrosis within the femoral head and a linear radiolucency around the stem indicated instability of the femoral component in addition to a chronic biomechanical fracture. There was no relationship between the depth of the cement penetration at the dome and the various failure patterns.
On the basis of the defined characteristics and the amount of osteonecrosis and vital reactive changes of bone tissue within the femoral remnants, three distinct fracture patterns (Fig. 1) were identified histologically:
Type A—acute biomechanical fracture (Fig. 2):
This fracture was characterized by diffuse reactive changes and varying degrees of perfusion of the proximal bone depending on the vascular injury resulting from the fracture. There was no evidence of osteonecrosis, regenerative fibrosis, or vascular proliferation. The fracture line was characterized by irregular fragments of osseous trabeculae, focal mechanical deformation, and destruction of bone marrow in addition to focal precipitates of calcium salts and the presence of bone chips.
Type B—acute postnecrotic fracture (Fig. 3):
This fracture was characterized by the association of the fracture line with osteonecrosis within the femoral remnant. The advanced osteonecrotic lesions demonstrated bordering fibrosis and sclerosis, focal resorption, and vascular proliferation. The bone trabeculae displayed empty osteocyte lacunae. Bone marrow showed a high degree of disorganization with focal saponification of necrotic fat cells. Several specimens demonstrated fibrosis directly under the area of intertrabecular cement interdigitation. Within the zone of fibrosis, appositional new bone formation had occurred in the absence of microscopically visible trabecular microfractures. Such postnecrotic changes bordering new bone formation may have been responsible for radiographically detectable linear sclerosis.
Type C—chronic biomechanical fracture (Fig. 4):
The fracture was characterized by the finding of refracture or pseudarthrosis within mineralized callus following a previous fracture. Discrete-to-moderate diffuse intramedullary fluid accumulation and histiocytic infiltration of the bone marrow of the proximal fragment, consistent with bone-marrow edema, was seen in the majority of cases.
We also categorized all of the specimens into two distinct macroscopic groups depending on the position of the fracture line with regard to the edge of the femoral component. The majority of the fractures (sixty-three; 59%) occurred in the bone inside the femoral component (in the femoral head), and the hips with this type of fracture failed significantly later than the forty-four hips in which the fracture occurred in the bone outside the component (in the femoral neck) (p < 0.0001) (Table I). It is noteworthy that acute biomechanical fractures were located exclusively in the femoral neck (p < 0.0001) in this study cohort.
A type-A (acute biomechanical) fracture occurred in nine hips (8%). This type of fracture was significantly less frequent than the type-B (acute postnecrotic) and type-C (chronic biomechanical) fractures (p < 0.001), and the hips with this type of fracture failed significantly earlier than did those with a type-B (p = 0.002) or type-C (p = 0.001) fracture (Table II).
Almost two-thirds of the type-B fractures (thirty-six; 66%) occurred in the bone inside the femoral component (p = 0.022). Similarly, twenty-seven (63%) of the type-C fractures were detected in the bone within the resurfacing cup (p = 0.093). The times to revision of the hips with the type-B fracture and those with the type-C fracture were similar (p = 0.263) (Table II).
Statistical analyses according to clinical diagnosis, sex, age, body mass index, device design, stem-shaft angle, and number of surgical procedures performed prior to the revision of the femoral component revealed no significant differences, on the basis of the numbers available, among the three morphologically defined fracture types.
Both intraobserver (Observer A) and independent interobserver (Observers A and B) variability testing revealed almost perfect agreement23 for the macroscopic diagnosis (Table III). There was almost perfect intraobserver agreement and both substantial and almost perfect interobserver agreement regarding the histopathological diagnosis (Table III).
In this study, we focused on identifying different patterns of periprosthetic fractures that occurred without antecedent trauma after total hip resurfacing arthroplasty. We hypothesized that changes in the mechanical competence of the proximal part of the femur due to the surgical procedure, biomechanical adaptations of the viable remaining portion of the femoral head, and pathological changes—particularly osteonecrosis—of the femoral remnant are the major predisposing factors for this complication. While all hips in the study cohort fractured without antecedent trauma, no other isolated causal factor for this complication was identified.
Acute biomechanical fractures occurred primarily within the first few weeks after implantation, which corresponds to the initial period of full loading after the index surgery; these fractures were located outside of the bone within the resurfacing cup—i.e., they were in the femoral neck. As the bone appeared viable histologically in these cases, we suggest that this fracture is caused by factors other than biological ones. We hypothesized that aspects of the surgical technique leading to changes in the biomechanical characteristics of the femoral neck may result in short-term acute failure due to mechanical weakening of the bone segment. There are several potential explanations for femoral neck weakening, such as notching or uncovering of reamed bone16. A thick cement mantle at the dome of the femoral head (observed in some of the specimens) also suggests neck lengthening resulting in higher bending moments as a possible cause. Recently, Steffen et al. analyzed femoral fractures after total hip resurfacing and did not find any substantial differences in the frequency of supposedly adverse mechanical factors such as postoperative neck lengthening, notching, or varus alignment of the femoral component14. However, as all of the specimens that were evaluated histologically in that study showed evidence of osteonecrosis14, these cases might better be defined as acute postnecrotic, and not biomechanical, with use of the above criteria and thus might have failed secondary to a cause other than biomechanical insufficiency.
Hips with less biomechanical insufficiency after total hip resurfacing may not fail as a result of an acute fracture but instead may fail as a result of a chronic fracture that has healed with the formation of callus or a pseudarthrosis, depending on the stability of the fracture. Recent experimental data showing that accumulative compressive bone damage can occur support this morphologic observation24.
Whether the initial event leading to some chronic biomechanical fractures can be linked to the index procedure remains questionable. Shimmin et al. reported that two of their first fifty fracture-related failures (in a series of 3497 surface replacements) were due to intraoperative fracture15, and Steffen et al. reported that two of twelve failures due to a femoral neck fracture (in a series of 610 arthroplasties) were due to an intraoperative femoral neck fracture8. On the basis of these experiences, it seems likely that an intraoperative fracture is not a frequent event; also, its occurrence usually leads to direct conversion to a total hip arthroplasty at the time of the index surgery. Our morphologic findings in hips with an acute biomechanical fracture are consistent with those of recent simulation and cadaver model studies, which demonstrated similar patterns of mechanically caused subcapital and transcervical fractures25,26.
Osteonecrosis of the femoral remnant may result in a different type of fracture that can be well defined histologically. The etiology of osteonecrosis after total hip resurfacing is not known. However, osteonecrosis has been observed even in hips with a histologically proven absence of osteonecrosis at the time of the index procedure27. Acute postnecrotic fractures occurred both at the line of demarcation of the osteonecrosis as well as proximal to it, findings reminiscent of the intralesional fractures seen with conventional osteonecrosis of the femoral head28. It is noteworthy that some of the postnecrotic fractures in our series occurred as late as two to three years after implantation. Even though these fractures occurred more frequently in the bone inside the femoral component, they can mimic chronic fractures when extensive osteonecrosis is evident radiographically. Interestingly, both of these fracture patterns occurred at a similar time after the index surgery. Therefore, histological analysis of femoral bone after failure appears to be important for distinguishing the different modes of periprosthetic fractures after total hip resurfacing. All fractures in the cohort analyzed here were intracapsular, and the fracture line always was in contact with the stem of the femoral component.
The most important diagnostic difficulties that we encountered were in the histopathological differentiation between acute postnecrotic and chronic biomechanical fractures that occurred in the bone within the femoral component at two to three months. At this time after the index surgery, osteonecrosis of the proximal segment might not be fully developed and initial bordering sclerosis with focal reactive new bone formation can mimic the formation of fracture callus.
We recognize several limitations to the present study. We obtained retrieval specimens along with basic clinical information from sixty-eight different medical centers from sixteen different countries. Because we probably received only a subset of the failures from each site, we could not analyze some specific issues such as the prevalence of fractures or the influence of the experience of the institution or the individual orthopaedic surgeon on the outcome of the procedure. Some of the clinical information, such as the type of the initial surgical approach or the duration of the index surgical procedure, was not available for all cases. Furthermore, several revisions in our cohort were performed by an orthopaedic surgeon other than the surgeon who carried out the index procedure.
To summarize, we suggest that weakening of the bone due to osteonecrosis and changes in the biomechanical properties of the hip following resurfacing arthroplasty (due to notching of the femoral neck, malpositioning such as varus alignment, or neck lengthening) are causes of periprosthetic femoral neck fractures.
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