Introduction

Acetabular cup positioning has been linked to dislocation and increased bearing surface wear. A previous study found correlations between patient and surgical factors and acetabular component position. The purpose of this study was to determine if acetabular cup positioning improves when surgeons receive feedback on their performance.

Purpose: The purpose of this study was to determine if correlation exists between acetabular cup positioning and factors relating to the surgeon and patient.

Method: Data for 2063 patients who underwent primary or revision THA from 2004 – 2008 were compiled. The post-op anteroposterior (AP) and cross-table lateral digital radiographs for each patient were obtained. The AP radiograph was measured using Hip Analysis Suite to calculate the cup abduction and version angles (version direction determined separately). Acceptable ranges were 35–45° for abduction, and 5–20° for version. Correlations were then determined with SPSS™ software.

Results: There were 1980(96%) qualifying patients. There were 1025(52%) acetabular cups that fell within the 35–45° abduction range, and 1287(70%) cups in the 5–20° version range. Regression analysis showed that the only independent predictor of acceptable abduction angle was the surgical approach (p< 0.001). Posterolateral approach was the most accurate (57% acceptability). In contrast to the posterolateral, the MIS (2 incision) approach was 3 times (95%C.I. 1.5–5, p=0.001), and the mini anterolateral approach 2.5 times (95%C.I. 1–6.5, p=0.035) more likely to have unacceptable abduction angles. The only independent predictor of acceptable version was the performing surgeon (p< 0.001), with higher volume surgeons showing greater accuracy.

Conclusion: The posterolateral approach was superior to MIS (2 incision) and mini anterolateral approaches for acceptable abduction angle, and surgeon volume influenced version angle acceptability. Further analysis on variables and their influence on cup position at a lower volume medical center would provide a valuable comparison.

There are a variety of patient and surgical factors shown to increase post-operative complication risk for a total hip arthroplasty (THA). While many studies have linked patient and surgical factors to unsuccessful outcomes post total hip arthroplasty (THA), no study has attempted to correlate the infiuence of these factors to the positioning of the acetabular cup. The purpose of this study was to determine if a correlation exists between patient and surgical factors and the anatomical position of the acetabular component.

Data for 2063 patients from 2004–2008 who underwent a primary total hip arthroplasty (THA), revision THA, or Birmingham Hip Resurfacing procedure was compiled. The post- op anteroposterior pelvis (AP) and the cross table lateral digital radiographs for each patient were measured to determine cup inclination and version. Acceptable angle ranges were defined as 30–45° for abduction, and 5–25° for version. Correlations between variables and cup abduction and version angles were determined with SPSS™ statistical software.

There were 1954(95%) qualifying patients. There were 1218(62%) acetabular cups that fell within the 30–45° optimal abduction range, and 1576(87%) cups in the 5–25° optimal version range. There were 921(47%) patients that had both inclination and version angles that fell within the optimal range. Regression analysis showed that surgical approach (p> 0.001), high/low volume surgeon (p< 0.001), and obesity (BMI > 30, p=0.01) were independent predictors for abduction and version combined analysis. Both surgical approach (p< 0.001) and BMI (p=0.018) were independent predictors in the individual analysis of both abduction and version. High/low volume surgeon was significant for the independent analysis of abduction (p=0.013). In the combined analysis, low volume surgeons showed a 2 fold increase (95% C.I. 1.5–2.8) in risk for cup malpositioning compared to high volume surgeons. The MIS surgical approach showed a 6 fold increase (95% C.I. 3.5–10.7) in risk for cup malpositioning compared to the posterolateral approach. Obesity (BMI> 30) showed a 1.3 fold increase (95% C.I. 1.1–1.7) in risk for cup malpositioning compared to all other body mass index groups.

Posterolateral surgical approach was superior to MIS surgical approaches for independent and combined abduction and version analysis. High volume surgeons had greater accuracy for cup positioning, specifically for achieving optimal cup abduction angle. Compared to all other body mass index categories, patients that were obese (BMI> 30) displayed a greater risk for cup malpositioning for independent and combined abduction and version analysis. Further statistical analyses on patient and surgical variables and their infiuence on cup position at a lower volume medical center would provide a valuable data comparison.

Studies of patients having primary THR using highly cross-linked polyethylene show excellent clinical outcomes and very low radiographic wear results at a minimum of 5 years follow-up. Recently, a radiostereometric analysis (RSA) study of a small group of patients reported that after no detectable wear during years 1–5, they found a significant increase in femoral head penetration between 5 and 7 years follow-up. However, this increase in head penetration after 5 years has not been confirmed in a larger patient cohort.

The purpose of this study was to organize a multicenter radiographic study involving leading medical centers in the U.S. having the longest-term follow-up available on this type of highly cross-linked polyethylene in order to determine if the RSA observation can be confirmed in a larger study.

Six academic centers agreed to contribute radiographic data to this study. All patients received primary total hip replacements with Longevity polyethylene liners (Zimmer, Warsaw, IN) coupled with 26, 28, and 32mm cobalt chrome femoral heads. The radiographic inclusion criteria required a minimum of four radiographs per patient: one at 1 year; at least one from 2 to 4.5 years; one 4.5 to 5.5 years; and at least one from 5.5 to 9 years follow-up. The Martell Hip Analysis Suit-eTM software was used for the wear analysis. All wear values were determined by calculating head penetration between the follow-up radiograph and the 1-year radiograph to remove creep, the majority of which has been shown to occur during the first year. Separate linear regressions, representing the wear rates, were computed for the early period from 1 year to 5.5 years and the late period from 5.5 years to 9 years follow-up. The Zar test was used to determine the significance of the difference between these two linear regressions.

We present the completed analysis of 165 hips. When the early and late data points were combined into one data set, the second-order regression indicated an inflection point at 6.3 years with a slightly positive inflection. There were 402 film comparisons in the early time period, and the slope and confidence interval of the regression line was 4.9μm/yr (95% CI of −28μm/yr to 38μm/yr). There were 188 film comparisons in the late period, and slope of the regression line for the late period was 10.8 μm/yr (95% CI of −58μm/yr to 80μm/yr). The Zar test showed no significant difference between the two slopes (Figure 1, p=0.886).

No significant increase in femoral head penetration was found for the late period after 5 years compared to the early period before 5 years follow-up in either analysis. Additionally, no significant late increase in wear was seen within individuals. While we continue to enroll patients, at this time we do not observe the increase in wear seen in the RSA study after 5 years.

Highly cross-linked polyethylene liners in total hip replacement (THR) have allowed the use of larger diameter femoral heads. Larger heads allow for increased range of motion, decreased implant impingement, and protection against dislocation. The purpose of this study is to report the clinical and radiographic outcomes of patients with large femoral heads with HXLPE at 5 years post-op.

A group of 124 patients (132 THRs) who had a primary THR with a 36mm or larger cobalt-chrome femoral head and a Durasul or Longevity liner (Zimmer; Warsaw, IN) were prospectively enrolled in this study. 93 THRs (88 patients) had minimum 5 year follow-up. All patients received a cementless acetabular shell (Trilogy or Inter-op, Zimmer Inc, Warsaw IN) and a highly cross-linked polyethylene liner with an inner diameter of 36 or 38mm. The median radiographic follow-up was 5.6 years (range 5.0–8.0), and patients were assessed clinically by Harris Hip score, UCLA activity score, EQ-5D, and SF-36 functional scores. Femoral head penetration was measured using the Martell Hip Analysis Suite.

No osteolysis was seen in the pelvis or proximal femur, and no components failed due to aseptic loosening. Four patients have questionable signs of bone changes around the acetabular shell with future CT scans scheduled to help reach a final determination. The median acetabular shell abduction and anteversion were 44° (30–66°) and 13° (3–33°) respectively. There was no evidence of cup migration, screw breakage, or eccentric wear on the liner. Regarding the femoral component, there were no episodes of loosening, migration, osteolysis, or fracture. There was no significant difference in the median penetration rate from post-op to longest follow-up between the 36mm (24 patients) and 38mm (4 patients) femoral head groups (0.056±0.10mm/yr and 0.060±0.05mm/yr respectively). Therefore, the data were pooled into one group. Using every post-op to follow-up comparison, the linear regression penetration rate of this combined group was 0.003 mm/yr which is within the error detection of the Martell method. The median femoral head penetration rate during the first post-op year measured 0.59±1.04 mm/yr. In contrast, the median steady state wear rate from the 1yr film to the longest follow-up measured -0.009±0.15mm/yr. A linear regression steady state wear rate from the 1 year film to every follow-up of −0.031 mm/yr indicated no correlation between the magnitude of polyethylene wear and time.

The mid-term results on this series of patients with THRs with a 36 or 38mm femoral head articulating with highly cross-linked polyethylene showed excellent clinical, radiographic, and wear results. The lack of early signs of osteolysis with the use of these large diameter femoral heads is encouraging. Continued and longer-term follow-up is needed to provide survivorship data.

Highly cross-linked polyethylene (HXLPE) is one of the most widely utilized bearing surfaces for total hip arthroplasty (THA). The first patients to receive XLPE will be 10 years post-op as of December 31, 2008. The purpose of this study is to report the long-term clinical and radiographic outcomes of patients implanted with HXLPE.

A group of 247 primary total hip replacements (224 patients) using HXLPE liners (Longevity or Durasul, Zimmer Inc.) with 22, 26, 28, or 32mm femoral heads were implanted between 1999 and 2001. Clinical evaluation measures included the Harris hip, EQ-5D, SF-36 functional scores, and UCLA activity scores. In addition to plain radiograph assessment, the computerized Martell method was used to measure head penetration over time. A matched group of 241 primary total hip replacements (201 patients) with the same head sizes using conventional polyethylene (PE) with a minimum of 7 years follow-up was used as a Martell method control group. The steady state penetration rate was defined as the slope of the linear regression line of the plot of head penetration from the 1 year film to each subsequent film to discount the early bedding-in process. A student’s t-test was used to compare wear rates between head sizes in each group, and a repeated-measures mixed model ANOVA was used to compare the groups for the 28mm head size.

There were no osteolytic lesions around the cup or stem, and no revisions were performed for polyethylene wear or liner fracture. Clinical outcome scores were averaged: Harris Hip 88.1±11.97, EQ-5D 74.0±27.0, SF-36 physical activity scores 53.3±8.4, SF-36 mental score 46.9±11.1, and UCLA activity 6.4±2.1. The steady state wear of the conventional polyethylene patients increased with time for both 26 and 28mm head sizes (0.144 and 0.127mm/year, respectively). No significant difference was found between the head sizes coupled with conventional polyethylene (p=0.14). Femoral head penetration in the highly cross-linked polyethylene did not increase over time after the first year. The steady state wear rates of HXLPE liners with 28mm or 32mm femoral heads were not significantly different than a slope of zero (p=0.54 for both head sizes).

Clinical follow-up results are typical of a primary THR patient population, and the radiographic results are excellent with no signs of peri-prosthetic osteolysis. Patients with PE show wear rates that are significantly different than zero indicating significant wear of the material. Conversely, patients with HXLPE display no measureable wear at 7–9 years as the wear rates were within the error detection of the Martell method. This long-term clinical and radiographic follow-up study for this new bearing material shows excellent clinical outcomes with very low in vivo wear.

Introduction: Electron beam irradiated highly cros-slinked polyethylene has been used in total hip arthroplasty for over 8 years. Due to its low wear characteristics, the use of femoral heads that are greater than 32mm in diameter is now available, allowing for an increase in range of motion and increased stability against dislocation when necessary. The purpose of this study is to provide a summary report on the radiographic analysis of the longest term data available on primary THR patients receiving highly cross-linked polyethylene and to compare the results of two methods of measuring femoral head penetration.

Methods: Three prospective clinical studies involving electron beam irradiated highly cross-linked polyethylene have been initiated at our center. To date, the results of: 200 hips with a minimum of 6 year follow-up with conventional sized femoral heads (primarily 28 and 32mm); 45 hips with minimum 5 year follow-up with larger sized femoral heads (primarily 36 and 38mm); and 30 hips with 5 year follow-up enrolled in a Radiostereometric analysis (RSA) study (15 patients with 28mm and 15 patients with 36mm diameter femoral heads); were available for this summary report. Data from patient administered questionnaires (Harris Hip score, UCLA activity score, and WOMAC), radiographic review, and wear analysis using RSA or the Martell Hip Analysis Suite™. In addition, for comparative purposes, wear was measured in a subset of patients using the Devane Polyware™ program.

Results: All hips had good clinical outcome at longest follow-up regardless of which femoral head size was used. There were no revisions due to polyethylene wear and no evidence of peri-prosthetic osteolysis. In general, after the bedding in period, there was no significant increase in femoral head penetration regardless of head size.

For RSA, the wear rate for the 28mm femoral head group was 0.05±0.02 while the 36mm femoral head group was 0.03±0.02, p=0.13.

For the Martell analysis, the average steady-state wear rate was −0.002 ± 0.01 mm per year and −0.026 ± 0.13 mm per year for 28mm and 32mm head sizes, respectively, p=0.62. There was no correlation between wear and time in situ or femoral head size for any of the clinical studies.

In comparing the Martell and Devane programs, the total average wear rates were significantly different, 0.07±0.05 and 0.03±0.06mm/year respectively, p=0.01. However, when the absolute values of the Martell results were used, there was no difference, p=0.22.

Conclusion: The mid-term follow-up of a large group of primary THR patients receiving highly cross-linked polyethylene components have shown no problems related to the new bearing material. Extremely low wear and lack of peri-prosthetic osteolysis are encouraging results requiring further long-term study.

Fifty-four cobalt-chrome alloy femoral heads were retrieved at revision surgery or at post-mortem; 23 came from uncemented, eight from hybrid and 23 from cemented hip arthroplasties. The uncemented and hybrid implants had porous coating, metal backing and modular femoral heads; with one exception none of the cemented implants had any of these. Twenty-five of the 31 heads from uncemented and hybrid arthroplasties, and 11 of the 23 heads from cemented arthroplasties showed surface damage involving more than 25% of the surface. Scanning electron microscopy revealed multidirectional fine scratches 1 micron to 10 microns in depth and width which appeared to have been made by fine, hard particles. There was a higher rate of such damage in the uncemented and hybrid arthroplasties than in the cemented implants, suggesting that the abrasive particles were mainly released from the metal, rather than from the cement or polyethylene components of the implants.

Six porous-coated, uncemented femoral components were revised at a mean of 34.5 months for persistent thigh pain. At operation the stems were rigidly stable, difficult to extract, and showed good bony ingrowth. The four men and two women, with an average age of 59 years, all had thigh pain starting within the first year, progressive over time and unresponsive to conservative measures. These cases show that rigid fixation with good bony ingrowth does not guarantee the clinical success of a porous-coated uncemented femoral stem.

The radiographic and histological features of radiolucent areas at the cement-bone interface were correlated in 15 specimens retrieved at post-mortem from patients who had undergone cemented total hip arthroplasty, two weeks to 15 years prior to death. All but one of the components were securely fixed, as demonstrated by direct measurements of micromotion. Extensive radiolucencies were present in all but one case. In 11 of the 14 specimens with radiolucencies, histological examination showed that the radiolucent areas represented regions of osteoporosis and bone remodelling. The remodelling changes were characterised by osteoporosis, cancellisation and thinning of the endosteal cortex, and osteopenia of the trabecular bone. In two specimens the appearance of radiolucency was found to be due to fibrous tissue at the cement-bone interface and in one specimen there was a mixed picture of osteolysis and fibrosis. The study demonstrates that radiolucent lines can occur with well-fixed components and that they may commonly represent osteoporosis rather than the presence of a fibrous membrane at the cement-bone interface.

We studied 16 femora retrieved at post-mortem from symptomless patients who had a satisfactory cemented total hip arthroplasty from two weeks to 17 years earlier, with the aim of delineating the initial mechanisms involved in loosening. Only one specimen showed radiographic evidence of loosening; the other 15 were stable to mechanical testing at 17.0 Nm of torque. In all 16 specimens, the cement-bone interface was intact with little fibrous tissue formation. By contrast, separation at the cement-prosthesis interface and fractures in the cement mantle were frequent. The most common early feature was debonding of the cement from the metal, seen at the proximal and distal ends of the prosthesis. Specimens which had been in place for longer also showed circumferential fractures in the cement, near the cement-metal interface, and radial fractures extending from this interface into the cement and sometimes to the bony interface. The most extensive cement fractures appeared to have started at or near sharp corners in the metal, or where the cement mantle was thin or incomplete. Fractures were also related to voids in the cement. The time relationship in this series suggested that long-term failure of the fixation of cemented femoral components was primarily mechanical, starting with debonding at the interface between the cement and the prosthesis, and continuing as slowly developing fractures in the cement mantle.