Widespread use of XLPE has reduced the incidence of both revision surgery and dislocations. This paper aims to create gross estimates of the magnitude of the resulting cost savings.

Data about decreasing rates of revision surgery and dislocations after THA, comparing XLPE versus CPE, were obtained from the literature along with figures for costs. Gross estimates were generated from these figures.

AOA NJRR 17 reports that at 15 years XLPE reduced the “all cause” revision rate of ceramic on polyethylene by 6.8% and at 16 years reduced the rate for metal on polyethylene by 6.5%. Their average 15 year figure was 4.8%, nearly in half. Dislocations were reduced by 24%. Other data substantiate these trends.

Inclusive costs of nonseptic revisions in US dollars, incorporating those relevant costs for the year prior to and the year following surgery, have been shown to average in the neighborhood of $55,000. Dislocations add further to the costs. The estimated savings from XLPE by 15 years on those total hip replacements done in one year in North America equal nearly $1 billion, even without incorporating any increase annually in the number done. While the specific savings in other countries with lower costs are less, similar proportions likely obtain. Despite all the assumptions, the magnitude is striking.

In addition to major benefits to the patients, the surgeons, and the hospitals, XLPE affords a massive advantage economically.

The extraordinary majesty of THR, as it burst onto the scene 60 years ago, both dazzled and blinded. It dazzled patients and surgeons alike and simultaneously obstructed a clear eyed assessment of the human costs. It behooves current practitioners, who have benefited mightily by our progress, to pause and reflect thoughtfully on that progress. Look no further than the fact that the treatment of a benign disease left one patient out of every 50 dead. Dead from a pulmonary embolus and that over 25% of the patients threw pulmonary emboli.

What were the big six major disadvantages: 1) Fatal pulmonary emboli; 2) Prosthetic joint infection; 3) Failure of fixation; 4) Dislocation; 5) Periprosthetic osteolysis; 6) Prolonged hospitalization.

Start with the observation that THR in the modern era began with Charnley's experiment with Teflon articulations. Of the nearly 300 such operations done, nearly 300 failed.

Ultrahigh molecular weight polyethylene was better- much better. But still it produced wear and periprosthetic osteolysis, afflicting an estimated 1 million patients. Periprosthetic osteolysis became the most common reason for failure, the most common reason for reoperation, the most common reason for fracture, and the most common reason for extremely difficult re-operations requiring major grafting.

Reoperation rates in certain series were 20 to 30% from loosening and 20 to 40% from osteolysis. Dislocation catapulted the unsuspecting patient to the floor at a rate of one out of 20 patients and the initial rate of prosthetic joint infection was 10%. Most patients were hospitalised in the new neighborhood of 2.5 weeks, at huge expense.

Massive progress has been made but forget not that this striking progress was not obsessively linear. Recall the recent, extraordinary and continuing massive failure of metal-on-metal total hip replacements, despite 40 prior years of experience, predicting that metal-on-metal total joints should be ‘just fine’.

Over the past six decades every one of the six major disadvantages listed above has been reduced by an order of magnitude. The challenge to you is to continue that progress.

Now that periprosthetic osteolysis is under control, analyzing how it was overcome has substantial value. What can we learn about how to attack the next enigma? There were six important generic drivers leading to that success:

Surmounting the failure to recognize its full nature. Why was it that the lapse between the first case and the publication of the article called “cement disease” was three decades? In part, because the concept of “cement disease” was simultaneously ingenious, essential, incomplete and misleading.

Understanding “biology beyond the histology.” Histology assured us that this destruction process was not malignant but well beyond that was discovery of the novel and exquisite understanding of the molecular biology.

The importance of multiple parallel approaches. The detailed molecular biology suggested that a pharmacologic prophylaxis should exist, but none was ever found. Of the three possible alternate bearings, the most likely (metal-on-metal) failed badly and the least likely (metal-on-polyethylene) succeeded.

The critical role of “beyond luck.” It is essential to understand more fully that serendipity nearly always follows Pasteur's motto that “chance favors only the prepared mind.”

The irreplaceable role of persistence. This is almost a homily.

The unavoidable requirement of risk-taking. Forty years of metal-on-metal THA without pseudotumors provided no protection against its current ravages.

Introduction

Total hip replacements using highly cross-linked polyethylene show excellent clinical outcomes, low wear, and minimal lysis at 5 years follow-up. A recent RSA study reports a significant increase in femoral head penetration between 5 and 7 years. This study is a multi-center radiographic analysis to determine whether the RSA observation is present in a large patient cohort.

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 (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.

Although complications associated with patello-femoral (PF) joint account for up to 50% of total knee replacement (TKR) revision procedures (Lee), the PF joint has been overlooked in wear simulations. The goal of this study was to develop an in vitro model to simulate patella wear in TKR’s. This report describes the concepts of an in vitro model for normal gait and the preliminary results of experimental validation.

The primary consideration in the development of the current model was modeling of the in vivo kinetics and kinematics. Since the in vivo kinetics are not well documented, the current model adapted a PF joint force pattern of gait measured one year postoperatively in a telemetric distal femoral replacement (Taylor et al). The maximum force was increased from 571N to 1780N (2.5xBody Weight) to compensate for muscle deficiency and to better reflect a maximum load representative of the in vivo situation. In vivo kinematics were adopted from measurements of Lafortune. Only the PF flexion was included in the model as a simplification of the complex patella motions. The phase relationship between the kinematic and kinetic waveforms was adjusted to replicate the in vivo situation. A 6-station knee simulator carried out the experimental validation with a test frequency of 1.5Hz. The test was intended to run for 5 million cycles, with CMM wear measurements (Muratoglu et al.) taken every million cycles. The preliminary measurements showed wear patterns in the tested patellae similar to retrieved patellae. Currently there are no standards for wear testing the PF joint. The current in vitro wear model presents a useful tool to critically assess the PF joint during gait. Future work should incorporate testing for adverse loading conditions, such as PF mal-alignment, rising from a chair or deep knee flexion.

A high proportion of complications following TKR occur at the patellofemoral articulation secondary to delami-nation and adhesive/abrasive wear. Electron beam cross-linking and melting has been shown to substantially reduce delamination and adhesive/abrasive wear in polyethylene tibial inserts. A series of in-vitro patella wear and fatigue tests were developed to explore the benefits of this material at the patellofemoral articulation.

Patellae (NKII, Sulzer Orthopedics, Inc., Austin, TX) were tested on an AMTI (Watertown, MA) knee simulator articulating against the trochlear grove of the femoral component. The simulator controlled flexion/ extension and patellofemoral contact force. Each test included patellae manufactured from conventional and electron beam crosslinked and melted polyethylene. Three different simulations were created: i) normal gait (5 million cycles) with optimal component alignment, ii) stair climbing (2 million cycles) with optimal component alignment, iii) stair climbing (2 million cycles) with 4° of femoral component internal rotation to simulate a component malalignment condition. In the last two simulations all patellae were artificially aged for 35 days in 80°C air to simulate one aspect of the long term oxidative state of each material.

In normal gait, the unaged conventional and highly cross-linked materials demonstrated similar behaviour. In stair climbing with optimal component alignment, the aged conventional patellae developed cracks by 2 million cycles. In stair climbing with component malalign-ment the aged conventional patellae developed cracks and delamination by 1 million cycles. None of the highly cross-linked components showed cracks or delamination. These results demonstrate the potential advantage of highly cross-linked polyethylene for the patella.

The purpose of this study was to compare the 2D and 3D linear and volume wear readings of the three most commonly used methods for measuring polyethylene wear: the Livermore, Devane and Martell techniques. Inter-observer variation of measurements using the techniques of Devane and of Martell on conventional radiographs was also performed. The radiographs of 80 patients (mean age 60+/−10 yrs) who had a Harris-Galante I total hip arthroplasty were measured. Nine different reviewers for the Devane technique readings including Dr Devane and eight reviewers for the Martell technique readings including Dr Martell made blinded independent wear observations for each radiograph set. One reviewer measured the 20 annual linear wear rate for all radiograph sets using the 2D Livermore technique. Inter-observer variation as a function of patient, reviewer, and total variation was statistically assessed using variance component analysis. Mean wear measured using the Livermore technique was the same as with the Devane and Martell method, but with a greater variation. Comparison of the Devane and Martell method for patient STD, reviewer STD, error STD (multiple reviews of same radiographs), total STD (randomly picked reviewer), mostly show a mean 50% lower STD with the Devane technique. Correlation (correlation coefficient of two randomly selected reviewers) is significantly better with the Devane technique.