LIMITED INFLUENCE OF PROSTHETIC POSITION ON ASEPTIC LOOSENING OF THE SOUTER-STRATHCLYDE TOTAL ELBOW PROSTHESIS

Transcription

1 6 LIMITED INFLUENCE OF PROSTHETIC POSITION ON ASEPTIC LOOSENING OF THE SOUTER-STRATHCLYDE TOTAL ELBOW PROSTHESIS J.C.T. van der Lugt R.B. Geskus P.M. Rozing Acta Orthop. 2005;76(5):654-61

2 Chapter 6 Abstract Introduction: Aseptic loosening of elbow replacements with long-term followup remains a problem. In this study, we attempted to ascertain the influence of cementing technique, prosthetic position, different component sizes, usage of a bone plug and intraoperative fractures on the development and progression of radiolucent lines and aseptic loosening. Methods: Standard radiographs of 125 primary Souter-Strathclyde total elbow prostheses were studied using the Wrightington method. Additionally, 104 preoperative radiographs were available for analysis. A Markow statistical model was used to detect relationships among all factors described above. Results: The mean followup of the radiographs was 67.4 months. Twenty-one prostheses loosened radiographically (16,8%) after an average period of 62.5 months (10-years survival 64,6 %). When the humeral component was tilted more medially or more anteriorly we found, respectively, the development of radiolucent lines at the medial condyle and at the posterior side of the humeral component. However, the progression of these lines was not influenced by these positions. No other prognostic factors on radiolucent lines or aseptic loosening were found. Interpretation: Despite the small number of elbows studied in this study, the poor influence of prosthetic position on aseptic loosening gives more ground for a multifactorial cause of aseptic loosening of the Souter-Strathclyde total elbow prosthesis. 86

3 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Introduction Total elbow replacement is a well-established treatment for the destructed elbow 1-6. The clinical results of most types of total elbow replacements are promising, but the survival rates are still not comparable to, for example, the survival rates for total hip replacements (THRs) 7. In recent literature 74 to 90 % of the implanted total elbow prostheses was still in situ after more than 10 years of followup 2,5,8. The most important factor that diminished the survival rate of elbow replacements was aseptic loosening of the humeral component Since the beginning of the 1980s we have used the Souter-Strathclyde total elbow prosthesis at our center, and we have also found high aseptic loosening rates of the humeral component 8,11. In this series of 125 elbow prosthesis we tried to determine different factors that affect aseptic loosening. First the survival of the 125 elbows is given for the endpoint radiological loosening. In most studies only the time to revision is taken to calculate the survival rate. Consequently, this would cause an underestimation of the actual survival rate, because not all loosened prostheses were revised immediately. Secondly the cementing technique of the prosthesis in the bone is analyzed. Little is known about the correct cementing technique, but the current opinion is that the prosthesis should be covered totally with a cement mantle of 1 mm thickness. The prosthetic position is of importance. Trail et al. used the Wrightington method for determination the prosthetic position and they found that more anterior tilting of the humeral component was related to aseptic loosening 12. We applied the same method in our series to obtain detailed information about the position of the prosthesis and the relationship to radiolucent lines and aseptic loosening during the followup. The recent tendency to use larger components of the Souter-Strathclyde prosthesis for primary surgery have improved the survival rates, but no scientific reason for this phenomenon has been stated 13. In this study the different humeral component sizes of the Souter-Strathclyde were related to the size of the patients humeral bone. The hypothesis is that a relative small prosthesis is fixed in cancellous bone, has therefore less cortical support and will, for that reason, loose more easily. Bone plugs have been proven in knees and hips arthroplasties, but the influence on elbow replacements is unclear One-third of the patients have received a humeral bone plug in this study and therefore this factor is taken separately into analysis. Intra-operative fractures may jeopardize the survival of prostheses 4,6. All fractures and their consequences at followup are described in this series. Finally, we have tried to determine the influence of preoperative radiographs on aseptic loosening. Unfortunately no standard scoring methods exists for the preoperative amount of bone resorption or dislocation of elbows. A new method was developed to accomplish this additional analysis. All data were analyzed using a Markov 87

4 Chapter 6 model, which was the first time this statistical model was employed for radiological evaluation of joint replacements Patients and Methods We retrospectively studied the standard radiographs of 125 primary Souter-Strathclyde total elbow prostheses (Stryker Howmedica Osteonics, Limerick, Ireland) inserted at our center. Standard radiographs were made as follows: the patient sits on a chair while resting the upper extremity on a table; the radiographs were made for the anteroposterior and lateral direction. For the anteriorposterior direction the shoulder is anteflexed and the elbow should be extended as much as possible and for the lateral view the elbow should be flexed 90 degrees whereas the shoulder is rotated internally. In this way the flexed elbow rests on the table (which consists the plate) in a horizontal position. All patients were observed for at least 2 years. The preoperative diagnosis was seropositive rheumatoid arthritis in 94 patients (108 elbows), seronegative rheumatoid arthritis in nine patients (nine elbows), juvenile chronic arthritis in two patients (three elbows), psoriatic arthritis in one patient (one elbow), synovial chondromatosis in one patient (one elbow), and posttraumatic osteoarthrosis in three patients (three elbows). The Larsen classification for the 120 elbows affected by rheumatoid arthritis was Larsen Class III in 10 elbows, Larsen Class IV in 62 elbows, and Larsen Class V in 48 elbows 20. The size of the components was determined before the operation by templating on radiographs. The size of the humeral components were small (48 elbows), medium (64 elbows), or large (13 elbows), and the ulnar components differed among small (66 elbows), medium (54 elbows), and metal- backed (five elbows, with the snap-fit option in four elbows) component. The latter component was used because of insufficient ulnar bone stock. When there was an additional tendency for dislocation after the trial reduction during the operation the snap-fit insert was used. We used the Wrightington method for assessing all radiographs (Fig. 1) 12. This method consists of 12 position measurements and 21 radiolucent line measurements. All measurements were done by the first author, who was not the operating surgeon, by using a universal goniometer. For assessing the position of the prosthesis we used only the first postoperative radiograph. All elbow prostheses were successively taken into analysis, but only if the first postoperative radiograph has been made more than 6 months after the operation (18 elbows) or when aseptic loosening was not the reason of failure (22 elbows) the elbow was excluded from this study. Radiolucent lines were defined as demarcation lines between the cement-bone interfaces seen on a standard 88

5 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Fig. 1. The Wrightington method for measuring radiolucent lines (RLLs) and position of the Souter-Strathclyde total elbow prosthesis. The RLLs are divided into zones and are scored in two directions (anterioposterior and lateral direction). radiograph and were classified as none, less than 1 mm, 1 to 2 mm, 2 mm, and greater than 2 mm. Loosening was defined as complete radiolucency (> 1 mm) around one or two components that was progressive in time. Cementing technique was classified as adequate as the thickness of the cement mantle was more than 1 mm and covered the prosthesis completely. When there was a defect in the cement mantle in one zone (see Wrightington method, Fig. 1) the mantle was classified as sufficient. In case of defects in more than one zone the cement mantle was classified as poor. For determination of the component size related to the size of the humeral bone two measurements were performed. First the distance between the medial and lateral condyle of the humeral bone, and secondly the distance between the prosthetic condyles of the humeral component were determined and, finally, the ratio of these two distances was calculated. In 46 elbows (37%) a humeral bone plug was applied and the influence of this factor on aseptic loosening was taken into analysis. Four intra-operative fractures were seen in this series and the consequences at postoperative outcome is discussed. Of the 125 elbows, 104 preoperative radiographs (made within 6 months before the operation) were available for additional analysis. First we classified bone resorption at four locations (medial and lateral condyles of the humerus, the olecranon, and the coronoid process), rated as none, minimal, moderate, or severe (Fig 2). Secondly, we measured narrowing of the humeroulnar joint space (in mm). Finally, horizontal and vertical dislocation of the elbow were measured (also in mm; Fig 3). All these observations were done by the first author and made on standard radiographs as described above. A Markov model was used to analyze the relationship between the position of the prosthesis and the development (from none to < 1 mm) or progression of radiolucent 89

6 Chapter 6 Fig. 2. Preoperative measurements of the horizontal (A.) and vertical dislocation (B.) on lateral view. Horizontal dislocation is the distance between the posterior cortex of the humerus and the most proximal cortex of the olecranon when the elbow is flexed 90 degrees. Vertical dislocation is the distance between the humero-ulnar joint surface and the lowest tip of the humeral medial condyle when the elbow is flexed 90 degrees. In elbows with severe destruction the dislocation increased in both directions. Fig. 3. The amount of bone resorption in the epicondyles, the olecranon and the coronoid process. coronoideus. The resorption is classified as non, minimal, moderate and severe. 90

7 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Fig. 4. Markov model to analyse the radiographs of the Souter-Strathclyde total elbow prosthesis. The RLL were analysed for different zones around the prosthesis according to the Wrightington method. Fig. 4. RLL = Radio Lucent Line(s) Fig. 4. a. Influence of position of the prosthesis on the development and progression of RLL Fig. 4. b. Development and progression of RLL Fig. 4. c. Influence of RLL on aseptic loosening Fig. 4. d. Influence of position of the prosthesis on aseptic loosening, independent on RLL lines (significance at p < 0.05 level; Fig 4). 1,2,24 The program we used allows the time of transition to the next category of radiolucent line width to be interval censored. This means that the exact transition time is only known to lie between two observation times. For the survival of the prosthesis (time to aseptic loosening), related to the preoperative radiographs and to the position of the prosthesis, we used a Cox model (significance at p < 0.05 level). The Kaplan-Meier survival curve was obtained for the overall survival of the prosthesis (end-point aseptic loosening). Results The mean followup of the radiographs was 67.4 months (range months, S.E. 3.6 months). Of the 125 elbow prostheses analyzed, 21 (16,8%) loosened radio- 91

8 Chapter 6 Fig. 5. Kaplan-Meier survival curve for endpoint aseptic loosening. The 95% confidence interval is given. graphically after an average period of 62.5 months (S.E. 7.3 months). In 17 elbows there was aseptic loosening of the humeral component, in one elbow there was aseptic loosening of the ulnar component, and in three elbows both components loosened simultaneously (Fig 5). The probability to remain free of aseptic loosening was 88.8 % (S.E. 3.3%) after 5 years and 64.6 % (S.E. 8.0%) after 10 years. No clear differences were seen in the cementing techniques. In all 125 elbows the cement mantle was 1 mm or more around both components directly after the operation and no cement defects were noticed; therefore, the quality of the cement mantles were classified as adequate. The position of components was not obvious related to radiolucent lines in this study. On anterioposterior view the humeral component was placed in lateral (102 elbows), neutral (21 elbows) or medial (2 elbows) position, whereas the ulnar component was placed in lateral (79 elbows), neutral (40 elbows) or medial position (6 elbows). On lateral view the humeral component was tilted anteriorly (61 elbows), posteriorly (30 elbows) or placed in neutral position (34 elbows) while the ulnar component was inserted in extension (99 elbows), flexion (7 elbows) or in neutral position. First we looked at the influence of position (as a continuous variable, in degrees) of the prosthesis on the development or progression of radiolucent lines in all Wrightington zones on AP and lateral views (see figure 1). Within one year after the replacement 92

9 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Fig. 6. Lateral and anteroposterior (AP) radiographs of two elbows with small humeral components taken 5 years after insertion. A. Lateral view of a well-fixed humeral component. B. Lateral view of a migrating humeral component with anterior tilting. C. AP view of a well-fixed humeral component D. AP view of a migrating humeral component (with a fracture of the medial epicondyle). The ulnar components in all figures show radiolucent lines, but no aseptic loosening was seen. 93

10 Chapter 6 Table I. Position Development radiolucent line Progression radiolucent line Humaral angle anterioposterior view Hap (S.E. 0.02), P= (S.E. 0.03), P=0.09 Hap (S.E. 0.02), P= (S.E. 0.03), P=0.49 Hap (S.E. 0.02), P= (S.E. 0.03), P=0.97 Hap (S.E. 0.02), P= (S.E. 0.03), P=0.95 Hap (S.E. 0.02), P= (S.E. 0.04), P=0.13 Humeral angle lateral view Hap (S.E. 0.04), P= (S.E. 0.06), P=0.42 Hap (S.E. 0.03), P= (S.E. 0.06), P=0.04 Hap (S.E. 0.03), P= (S.E. 0.04), P=0.73 Hap (S.E. 0.03), P< (S.E. 0.04), P=0.37 Hap (S.E. 0.04), P= (S.E. 0.06), P=0.71 Humeral angle anterioposterior view Hlat (S.E. 0.02), P= (S.E. 0.04), P=0.01 Hlat (S.E. 0.03), P= (S.E. 0.03), P=0.40 Hlat (S.E. 0.02), P= (S.E. 0.03), P=0.70 Hlat (S.E. 0.02), P= (S.E. 0.02), P=0.81 Hlat (S.E. 0.03), P= (S.E. 0.03), P=0.07 Humeral angle lateral view Hlat (S.E. 0.04), P= (S.E. 0.06), P=0.28 Hlat (S.E. 0.04), P= (S.E. 0.06), P=0.54 Hlat (S.E. 0.04), P= (S.E. 0.04), P=0.62 Hlat (S.E. 0.03), P= (S.E. 0.04), P<0.01 Hlat (S.E. 0.03), P= (S.E. 0.05), P=0.18 The relationship between the angle of the humeral component in the bone (as a continuous variable) and the development en progressional of radiolucent lines. On anteriorposterior view the humeral angle is given from valgus (positive value) to varus postion (negative value). On lateral view the humeral angle is given from posterior tilting (postive value) to anterior tilting (negative value). For all relationships the standard errors (S.E.) as well as the level of significance are given. Significance at p<

11 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Table II. Angle of components Humeral component Anterioposterior angle Lateral angle Ulnar component Anterioposterior angle Lateral angle Relation to aseptic loosening (S.E. 0.04), P= (S.E. 0.07), P= (S.E. 0.07), P= (S.E. 0.07), P=0.15 The relationship between the angle of the components (according to the Wrightington method, see figure 1) and aseptic loosening. The angles have been taken sa continuous variables. For all relationships the standard errors (S.E.) as well as the level of significance are given. Significance at p<0.05. surgery RLLs were found in 17,6 percent of the humeral and in 29,6 percent of the ulnar components. At latest followup these percentages increased respectively to 80,0 and 76,0 percent. When the humeral component was tilted more medially ( valgus position on the AP view) we found the development of radiolucent lines at the medial condyle and at the posterior distal zone (Hap 4, p = 0.02 and Hlat3, p=0.0, see table I). On the other hand, the progression of these lines was not influenced by this position. The same trend was seen for the development of radiolucent lines on the anterior and posterior side of the humeral component on the lateral view. If the humeral component was positioned in a more anterior tilting, radiolucent lines occurred more often in Hap4 (p=0.02) and Hlat2 (p=0.03), but the progression of these radiolucent lines did not depend on the prosthetic position. In contrast in three zones only progression of radiolucent lines was noticed whereas no influence on development of lines was seen in the same zones. For all other humeral and ulnar zones we could not demonstrate any other clear effect of the position of the humeral component on radiolucent lines. Even no effect of the position was found on the AP view after combining the radiolucent lines of zones Hap2 en Hap3 (lateral side of the prosthesis) and the humeral zones Hap4 and Hap5 (medial side of the component). In 69 elbows (55,2 %) radiolucent lines were seen around the ulnar component in Ulat4 en Ulat5 immediately after the operation, but also this finding had no effect on the progression of radiolucent lines or aseptic loosening in this study. We analyzed the relationship between the position of the humeral and ulnar components as categorical variables and radiolucent lines, but the outcomes were not divergent. The relationship between the position (also as a continuous variable, in degrees) of the prosthesis and aseptic loosening was analyzed, independent of the occurrence of radiolucent lines (Table II). The position of the components differed, but no prediction 95

12 Chapter 6 could be made on aseptic loosening. If we took the humeral angle on the lateral view as a categorical variable (anterior, neutral, or posterior position) instead of a continuous variable (in degrees), the outcome also did not change. A valgus or a varus position of the humeral component also did not have predictive value for aseptic loosening for continuous (in degrees) and categorical analysis, but only two humeral components were placed in medial position on AP view. None of the ulnar positions enhanced the risk of aseptic loosening. We measured the distances between the medial and lateral cortices of the epicondyles and the humeral component (humeral distances 1 and 2 on the AP view, Fig 1) and the ratio was calculated. However, although this ratio did differ in the 125 elbows (ratio range, ), we could not demonstrate a relationship to aseptic loosening. Intraoperative fractures occurred in four elbows. In three elbows the medial component fractured and in one elbow the lateral condyle of the humeral bone fractured. The peroparative alignment of these four prostheses had not been changed. Two of these elbows with intraoperative fractures had aseptic loosening (after 99 and 36 months), but both fractures had healed within 3 months after the surgery. For that reason an evident relationship between the fractures and aseptic loosening was not supposed. Three medial supracondylar ridges fractured after a average period of 31 months postoperatively (range months) after migration of the humeral component. In all these patients anterior migration (tilting) of the humeral tip had already occurred. The humeral bone plug did not prevent the occurrence of radiolucent lines in our series. No differences could be found in radiolucent lines or aseptic loosening between elbows with and without a humeral bone plug. Assessment of preoperative radiographs of the elbow had restricted weight in the prediction of aseptic loosening (Table III). First, the Larsen score did not predict the elbows which were at risk for aseptic loosening and, secondly, on our own scoring method could only prove that bone resorption around the lateral condyle was related (p = 0.02) to aseptic loosening of the prosthesis. None of the other items measured on the preoperative radiographs had an influence on aseptic loosening. Discussion The survival rate was approximately 10% lower after 5 and 10 years of followup when radiographic loosening, as previously defined, was used as the end point, compared with when the removal of the prosthesis was used as the end point 4,8. This can be explained by the absence of pain while the prosthesis loosened and, for that reason, the delay of the revision operation. 96

13 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis Table III. Preoperative data Ulno-humeral joint space Resorption medial condyle Resorption lateral condyle Resorption olecranon Resorption coronoid process Horizontal dislocation Vertical dislocation Aseptic loosening (S.E. 0.39), P= (S.E. 0.34), P= (S.E. 0.32), P= (S.E. 0.38), P= (S.E. 0.41), P= (S.E. 0.08), P= (S.E. 0.06), P=0.52 The relationship between preopartive radiographs and septic loosening. Narrowing of the preoperative joint space, increased preoperative resorption of medial condyle, olecranon, coronoid process or increased preoperative dislocation was not significantly related. Only preoperative resorption of the lateral condyle showed a significant relationship to aseptic loosening. For all relationships the standard errors (S.E.) as well as the level of significance are given. Significance at p<0.05. Making measurements on standard radiographs, as is done frequently in the outpatient clinic, has some pitfalls. The doctor who does the assessment should realize that such measurements depend on the rotation angle of the upper extremity and on the quality of the radiograph. The Wrightington method was developed to standardize measurements on standard radiographs of the Souter-Strathclyde total elbow prosthesis, but this method requires standarized radiographs of good quality. In contrast to Trail et al our series showed only minor influence of prosthetic position on aseptic loosening, maybe because the placement of most prostheses was around the average level and abnormal situations were hardly seen 12. We could postulate that the extremely divergent positions of the prosthesis could be related to aseptic loosening. Hermann and Egund found that the position of hip prostheses can be accurately and reproducibly ascertained from routine radiographs 21. Furthermore Pollock et al showed that liner thickness of acetabular components can be adequately determined on radiographs 22. Therefore we argue that measuring component-position on standard radiographs is also adequate for clinical assessment of elbow prostheses but, in our opinion, more detailed position measurements (migration up to 0.4 mm) should be performed with the RSA method in of a universal goniometer (with a 1-mm scale). Valstar et al showed that, with the application of radiostereometry (RSA) in cases of humeral loosening, the tip of the humeral component migrated anteriorly (anterior tilting). Unfortunately they did not look at the development and progression of RLLs in relation to this anterior tilting

14 Chapter 6 Radiolucent lines are difficult to interpret. The definition varies and, furthermore, the meaning of radiolucent lines is not clear 23,24. As in hip replacements, a radiolucent line around an elbow prosthesis will not always be the same as loosening 24. The value of radiolucent lines is restricted for this reason. The Markov model allows use to intergrate all different factors in one model. Because of that the Markov model provides a useful way to analyze the complex process of aseptic loosening. Weakness of our study is the relative small amount of elbows related to the large number of different factors. On the other hand, even no obvious statistical trend was seen after analysing 125 elbows. Probably the patterns of development and progression of RLLs and aseptic loosening are not as clear as one may have thought. The use of a bone plug in elbow prostheses is not well studied. Whereas in hip and knee replacements the efficacy of a bone plug has been proven (better cement mantle and less blood loss), the effect of bone plugs in elbow replacements is not well established In this series the cement mantle was of similar quality in all elbows and could therefore not predict aseptic loosening. In the preoperative radiographs, the elbows we analyzed were all destroyed by rheumatoid arthritis. The frequently applied Larsen score cannot classify the amount of dislocation or bone loss (resorption) before the operation 20. Because of this we developed an additional scoring list. More validated bone resorption-scores are necessary for research purposes and to understand the value of bone resorption prior to joint replacement surgery. Fracturing the medial epicondylar ridge during migration of the humeral component should be prevented. When the migration becomes obvious (progression of complete radiolucent lines) revision must be considered if fracture of the epicondyle is imminent. The preoperative radiologic scoring might be of importance, but could not be answered by this study. The factors that cause aseptic loosening of the primary Souter-Strathclyde total elbow prosthesis remain uncertain after preforming this study, but despite the relative small number of elbows, a multifactorial cause of aseptic loosening seemed to be most plausible. 98

15 Limited influence of prosthetic position on aseptic loosening of the Souter-Strathclyde Total Elbow Prosthesis References 1. Ferlec D C Total elbow arthroplasty for treatment of elbow arthritis. J Shoulder Elbow Surg 1999; 8(4): Gschwend N, Scheier N H, and Baehler A R. Longterm results of the GSB III elbow arthroplasty. J Bone Joint Surg (Br) 1999; 81(6): Mackay D C, Hudson B, Williams J R. Which primary shoulder and elbow replacement? A review of the results of prostheses available in the UK. Ann R Coll Surg Engl 2001; 83(4): Rozing P. Souter-Strathclyde total elbow arthroplasty. J Bone Joint Surg (Br) 2000; 82(8): Tanaka N, Kudo H, Iwano K, Sakahashi H, Sato E, Ishii S. Kudo total elbow arthroplasty in patients with rheumatoid arthritis. J Bone Joint Surg (Am) 2001; 83(10): Trancik T, Wilde A H, Borden L S. Captillocondylar total elbow arthroplasty. Two to eight year experience. Clin Orthop 1987; 233 : Herberts P, Malchau H. Long-term registration has improved the quality of hip replacement: a review of the Swedish THR Register comparing 160,000 cases. Acta Orthop Scand 2000; 71(2): Van der Lugt J C, Geskus R B, Rozing P M. Primary Souter-Strathclyde total elbow prosthesis for rheumatoid arthritis. J Bone Joint Surg (Am) 2004; 86 (3): Hargreaves D, Emery R. Total elbow replacement in the treatment of rheumatoid disease. Clin Orthop 1999; 366: Kudo H, Iwano K. Total elbow arthroplasty with a non-constrained surface-replacement prosthesis in patients who have rheumatoid arthritis: a long- term follow-up study. J Bone Joint Surg (Am) 1990; 72(3): Valstar E R, Garling E H, Rozing P M. Micromotion of the Souter-Strathclyde total elbow prosthesis in patients with rheumatoid arthritis: 21 elbows followed for 2 years. Acta Orthop Scand 2002; 73(3): Trail I A, Nuttall D, Stanley J K. Survivorship and radiological analysis of the standard Souter- Strathclyde total elbow arthroplasty. J Bone Joint Surg (Br) 1999; 81(1): Ikävalko M, Belt E A, Kautiainen H, Lehto M U. Revisions for aseptic loosening in Souter-Strathclyde elbow arthroplasty. Incidence of revision of different components used in 522 consecutive cases. Acta Orthop Scand 2002; 73(3): Klapach A S, Callaghan J J, Goetz D D, Olejniczak J P, Johnston R C. Charnley total hip arthroplasty with use of improved cementing techniques: a minimum twenty-year follow-up study. J Bone Joint Surg (Am) 2001; 83(12): Kligman M, Roffman M. The effect of intramedullary bone plug on femoral stem migration of hydroxyapatite-coated hip arthroplasty. Orthopedics 2000; 23(7): Kumar N, Saleh J, Gardiner E, Devadoss V G, Howell F R. Plugging the intramedullary canal of the femur in total knee arthroplasty: reduction in postoperative blood loss. J Arthroplasty 2000; 15(7): Andersen P K. Repeated assessment of risk factors in survival analysis. Stat Methods Med Res 1992; 1(3): Commenges D. Inference for multi-state models from interval-censored data. Stat Methods Med Res 2002; 11(2): Van Houwelingen H C. The future of biostatistics: expecting the unexpected. Stat Med 1997; 16(24): Larsen A, Dale K, Eek M. Radiographic evaluation of rheumatoid arthritis and related conditions by standard reference films. Acta Radiol Diagn (Stockh) 1997; 18(4): Hermann K L, Egund N. Measuring anteversion in the femoral neck from routine radiographs. Acta Radiol 1998; 39(4): Pollock D, Sychterz C J, Engh C A. A clinically practical method of manually assessing polyethylene liner thickness. J.Bone Joint Surg (Am) 2001; 83(12): Freeman M A. Radiolucent lines: a question of nomenclature. J Arthroplasty 1999; 14(1): Kwong L M, Jasty M, Mulroy R D, Maloney W J, Bragdon C, Harris W H. The histology of the radiolucent line. J Bone Joint Surg (Br) 1992; 74(1):

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