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Hip replacements
2 Hip replacements B. M. W R O B L E W S K I, P. D. S I N E Y and P. A. F L E M I N G, The John Charnley Research Institute, UK
Abstract: The Charnley low frictional torque arthroplasty (LFA) has now reached 45 years of continuous clinical success. Pain relief and excellent range of movements have been achieved and maintained over the follow-up time, but consumer demands for new products have stimulated the introduction of new materials and methods. The cemented Charnley LFA has withstood the test of time. The method offers a ‘customised’ prosthesis in every case but places the onus on the understanding and skill of the surgeon. Long-term results are results in young patients. The patient’s activity level is not a characteristic of any hip design or any method of component fixation, but a reflection of the patient selection. The immediate clinical success of hip replacement and the pressure to satisfy the increasing demand, especially in young patients, have highlighted the failure to understand the need for regular follow-up and the provision of ‘after-sales service’. The timing, indication and detailed operative findings at revision surgery have become important issues. Delays in revision surgery lead to progressive loss of bone and more complex technical problems. Regular follow-up after hip replacement with good-quality radiographs is essential. Cemented total hip arthroplasty continues to be an excellent method of treatment but, for young patients in particular, it is merely the beginning of the treatment. Key words: Charnley, long-term results, wear and loosening of the cup, strain shielding of the proximal femur, young patients.
2.1
Introduction
The success of total hip arthroplasty (THA) as a method of treating hip joints destroyed by arthritis has resulted in an increasing demand from an ageing population and has also extended the indications for this procedure. Consumer demand for new products stimulated the introduction of new materials and methods. Anecdotal, single-case, successes attract younger and less disabled patients. Immediate clinical success, short-term planning and pressure to satisfy the increasing demand, do not usually take into consideration the need for regular follow-up: the provision of ‘after-sales service’. 41
42
Orthopaedic bone cements
The availability of a successful ‘replacement’ procedure has also had a negative effect on the systematic study of underlying hip pathologies. It is not readily appreciated that clinical success is primarily due to pain relief. Pain relief is due to the natural, symptomatic joint being replaced with a mechanical neuropathic spacer; the whole construct functioning within a foreign body bursa. Wroblewski et al. (1999), in their long-term follow-up paper, stated that ‘clinical results do not reflect the mechanical state of the arthroplasty’ and that good-quality radiographs offer more information. Wroblewski et al. (2007a) also state that the activity level achieved after the operation is not a characteristic of a particular design, nor the method of component fixation, provided the fixation is secure; but it is a reflection of patient selection for the operation, patients with multiple disabilities never feature as anecdotal, single-case successes.
2.2
General principles
The method of treatment can be suitably reviewed under three main headings: design, materials and surgical technique.
2.2.1 Design The aim must be a set of components that allow a range of movements comparable with the natural joint, while ensuring stability without constraint. Any design must also take advantage of the best possible arrangements and combinations with respect to friction, frictional torque, wear and load transfer from the implant to the skeleton.
2.2.2 Materials Any combination of materials must be compatible with respect to each other and biocompatible with the tissues, not only in the solid but especially in their particulate form. They must also be fatigue and wear resistant.
2.2.3 Surgical technique Exposure must give an unimpeded, circumferential view of the acetabulum and the access to the medullary canal, in the area of the piriformis fossa, while preserving the integrity of the neighbouring structures as well as the all-important abductor muscle mass. Exposure of good-quality cancellous bone, pulse lavage, drying, containment, pressurisation of cement and correct timing of component insertion, are integral parts of the cementing technique.
Hip replacements
2.3
43
Bone cements
The characteristics of the various bone cements are well documented by Kühn (2000). Two aspects are of clinical importance: shrinkage on polymerisation and release of antibiotics. Shrinkage on polymerisation is the characteristic of the monomer. Our findings suggest that shrinkage occurs after setting/polymerisation has apparently taken place. The changes observed indicate that shrinkage would be away from both bone and the prosthesis. Under operative conditions the effect of the components acting as a ‘heat sink’ and intrusion into cancellous bone resisting the shrinkage, are yet to be quantified. Measured volume reduction is 25% (unpublished data), close to the 21% reported by Kühn (2000). Elution from polymerised cement is purely a surface phenomenon (Fig. 2.1). Wroblewski (1977), in his experimental evaluation, states ‘the volume released is proportional to the surface area of the cement mass’. Elution of gentamicin is neither continuous nor complete. Wroblewski et al. (1986) also noted that ‘on average 22% of the antibiotic is released within days of the operation’. Some of the remaining 78% will be released if exposure of the new cement surface occurs, as in cases of component loosening or cement fragmentation at a revision. This may affect samples taken for bacteriology.
2.1 A mixture of bone cement and gentian violet immersed in commercial bleach for 2 years. Note the thin outer bleached layer, some extensions of the process into the cement irregularities and the persistence of the dye within the cement mass.
44
Orthopaedic bone cements
2.4
Fixation of components with cement
Charnley (1963) in his lecture at the Société Internationale de Chirurgie Orthopédique et de Traumatologie (SICOT) said ‘The crux of the operation lies in the use of cement. By means of cement the load of the body weight is distributed over a large area of bone.’ This statement summarises succinctly all aspects of the technique. Charnley (1964) also emphasised that ‘Acrylic cement does not adhere to bone like glue, it merely forms an accurate cast of the interior of the bone so that load is transmitted evenly over all parts of the interface between cement and the cancellous bone’. This is the only method that offers immediately a ‘customised’ prosthesis in every case but also places the onus on the understanding and skill of the surgeon to achieve that aim. That skill is acquired by practice and is not a commodity that can be sold at a profit. This would probably explain the proliferation of ‘cementless’ designs. The terminology does not offer an indication of the method of component fixation except by stating what it is not, i.e. not with cement. It also hints that use of cement may be detrimental. Ultimately such designs become saleable commodities and take the responsibility for the quality of component fixation away from the surgeon and place it on the patient’s skeleton.
2.5
Long-term results
The Charnley low frictional torque arthroplasty (LFA) has now reached 45 years of continuous clinical application and must, therefore, be accepted as the gold standard (Wroblewski and Siney 1993).
2.5.1 Clinical results Using the Charnley LFA, pain relief is achieved and maintained; 96% of patients have either a totally pain-free hip or have no more than an occasional discomfort. Activity level, although improved and maintained, clearly depends on the underlying pathology and any factors that may become significant with time. Excellent, functional range of movements is achieved in 78% of patients and is maintained over the follow-up time.
2.5.2 Survivorship analysis Survivorship analysis used routinely in many hip registers continues to be the standard method of presenting long-term results with revision as the end point; each revision documented under a single indication (Soderman 2000). Our practice is to document all findings at revision (apart from infection which is a single finding). This gives a higher number of findings than
Hip replacements
45
Table 2.1 Survivorship analysis of 22 066 LFAs with a follow-up to 36 years
Reason for revision
Percentage success rate at 36 years’ follow-up
95% confidence limits
Infection Dislocation Fractured stem Loose stem Loose cup
91.4 98.0 88.6 72.5 48.6
100–67.9 100–85.9 100–62.4 100–39.2 79.1–18.0
hips revised and offers more valuable information (Wroblewski et al. 2007b). Survivorship analysis of the Charnley LFA, with a follow-up to 36 years is shown in Table 2.1.
2.6
Long-term problems
Two main long-term problems have been identified and addressed: (a) wear and loosening of the ultra-high molecular weight polyethylene (UHMWPE) cup and (b) strain shielding of the proximal femur. Charnley and Halley (1975) anticipated wear and loosening of the UHMWPE cup. They stated that ‘more than 5 mm wear might cause impingement of the neck of the prosthesis against the inner rim of the socket and cause loosening of the cement–bone bond in the acetabulum.’ The exponential correlation between wear and cup loosening has been documented in more recent studies (Wroblewski 1985a, Worblewski and Siney 1992, Worblewski et al. 2002). Issac et al. (1992) and Goldsmith et al. (2001), in their clinical and tribological studies, have identified factors affecting wear. The sequence of events of progressive penetration of the cup is restriction of movements, impingement of the neck of the stem on the rim of the cup and cup loosening. Reducing the diameter of the neck of the Charnley stem from 12.5 mm to 10 mm has delayed the impingement by the equivalent of 2 mm cup penetration. Clinical experience over the past 23 years (Wroblewski et al. 2004) supports the findings of the original theoretical model that used acrylic casts and shadowgraph techniques to measure real and radiographic wear (Wroblewski 1985b). Alumina ceramic/cross-linked polyethylene combinations, in the Charnley LFA, have now reached 20 years of clinical application (Wroblewski et al. 2005). The mean penetration rate is 0.02 mm/year with total penetration to date not exceeding 0.41 mm. Strain shielding of the proximal femur has been identified as being due to distal stem support not allowing proximal load transfer. The continuing developmental design of the stem, the C-Stem (DePuy International,
46
Orthopaedic bone cements
Leeds, UK) takes advantage of the common engineering principle: that of male (stem) and female (cement) tapers engaging under load. A number of factors essential for the concept to function have been detailed (Wroblewski et al. 2001). These are: a polished stem with a continuous triple taper; absence of distal stem support; and attention to the anatomical calcar. Clinical experience over the past 13 years has shown 100% survivorship – the end-point being stem loosening or revision – with 22% showing improvement in the radiographic appearance of the quality of the proximal femur.
2.7
Future trends
The cemented Charnley LFA has withstood the test of time. As with any THA, clinical results may not reflect the mechanical state, hence follow-up by good-quality radiographs is essential. Postal or telephone methods of collecting information are of limited value. Progressive loosening is an indication for revision, irrespective of the clinical result. This aspect must be understood and accepted by the patient, the surgeon and the system funding such a method of treatment. This must be agreed before the operation. Delays in revision surgery lead to progressive loss of bone stock and more complex technical problems to be tackled at revision. Long-term results are results in young patients. Increasing follow-up identifies ever-younger patients at the time of the operation. Now approaching 40 years’ follow-up, the mean age of patients at the time of the primary operation was 35 years. These are the results that have already been achieved with the Charnley LFA. Any newly proposed method or materials claiming to be suitable for young patients has the impossible task of providing data to support the statement that it will: accept patients with a mean age of 35 years and show a functioning arthroplasty when the patients reach the age of 75 years. This will not be achieved by a single generation of surgeons. Even then, continuity of design, materials and technique must be maintained. THA in general, and the Charnley LFA in particular, continue to be an excellent method of treatment, but for a young patient it merely heralds the beginning of treatment.
2.8
References
charnley j. 1963. Low friction arthroplasty of the hip in rheumatoid arthritis. SICOT Congress, Vienna, pp. 168–70. charnley, j. 1964. The bonding of prostheses to bone by cement. J Bone Joint Surg Br, 46, 518–29. charnley, j. & halley, d. k. 1975. Rate of wear in total hip replacement. Clin Orthop Relat Res, Oct (112), 170–9.
Hip replacements
47
goldsmith, a. a., dowson, d., wroblewski, b. m., siney, p. d., fleming, p. a. & lane, j. m. 2001. The effect of activity levels of total hip arthroplasty patients on socket penetration. J Arthroplasty, 16 (5), 620–7. isaac, g. h., wroblewski, b. m., atkinson, j. r. & dowson, d. 1992. A tribological study of retrieved hip prostheses. Clin Orthop Relat Res, Mar (276), 115–25. kühn k. d. 2000. Bone Cements. Up to Date Comparison of Physical and Chemical Properties of Commercial Materials. Springer, Berlin. soderman p. 2000. On the validity of the results from the Swedish National Total Hip Arthroplasty Register. Acta Orthop Scand, 71 (suppl 296), 4–33. wroblewski, b. m. 1977. Leaching out from acrylic bone cement. Experimental evaluation. Clin Orthop Relat Res, May (124), 311–2. wroblewski, b. m. 1985a. Charnley low-friction arthroplasty in patients under the age of 40 years. In Sevastik J. & Goldie, I. (eds), The Young Patient with Degenerative Hip Disease. Almquist and Wiksell, Stockholm, pp. 197–201. wroblewski, b. m. 1985b. Direction and rate of socket wear in Charnley low-friction arthroplasty. J Bone Joint Surg Br, 67 (5), 757–61. wroblewski, b. m., esser, m. & srigley, d. w. 1986. Release of gentamicin from bone cement. An ex-vivo study. Acta Orthop Scand, 57 (5), 413–4. wroblewski, b. m. & siney, p. d. 1992. Charnley low-friction arthroplasty in the young patient. Clin Orthop Relat Res, Dec (285), 45–7. wroblewski, b. m. & siney, p. d. 1993 Charnley low-friction arthroplasty of the hip. Long-term results. Clin Orthop Relat Res, Jul (292), 191–201. wroblewski, b. m., fleming, p. a. & siney, p. d. 1999. Charnley low-frictional torque arthroplasty of the hip. 20-to-30 year results. J Bone Joint Surg Br, 81 (3), 427–30. wroblewski, b. m., siney, p. d. & fleming, p. a. 2001. Triple taper polished cemented stem in total hip arthroplasty: rationale for the design, surgical technique, and 7 years of clinical experience. J Arthroplasty, 16 (Suppl 1), 37–41. wroblewski, b. m., siney, p. d. & fleming, p. a. 2002. Charnley low-frictional torque arthroplasty in patients under the age of 51 years. Follow-up to 33 years. J Bone Joint Surg Br, 84 (4), 540–3. wroblewski, b. m., siney, p. d. & fleming, p. a. 2004. Reduced diameter of the neck and its effect on the incidence of aseptic cup loosening in the Charnley LFA. J Bone Joint Surg Br, 87 (Suppl 1), 43. wroblewski, b. m., siney, p. d. & fleming, p. a. 2005. Low-friction arthroplasty of the hip using alumina ceramic and cross-linked polyethylene. A 17-year follow-up report. J Bone Joint Surg Br, 87 (9), 1220–1. wroblewski, b. m., siney, p. d. & fleming, p. a. 2007a. Charnley low-frictional torque arthroplasty in young rheumatoid and juvenile rheumatoid arthritis: 292 hips followed for an average of 15 years. Acta Orthop, 78 (2), 206–10. wroblewski, b. m., siney, p. d. & fleming, p. a. 2007b. Charnley low-friction arthroplasty: Survival patterns to 38 years. J Bone Joint Surg Br, 89, 1015–8.
Abstract: The Charnley low frictional torque arthroplasty (LFA) has now reached 45 years of continuous clinical success. Pain relief and excellent range of movements have been achieved and maintained over the follow-up time, but consumer demands for new products have stimulated the introduction of new materials and methods. The cemented Charnley LFA has withstood the test of time. The method offers a ‘customised’ prosthesis in every case but places the onus on the understanding and skill of the surgeon. Long-term results are results in young patients. The patient’s activity level is not a characteristic of any hip design or any method of component fixation, but a reflection of the patient selection. The immediate clinical success of hip replacement and the pressure to satisfy the increasing demand, especially in young patients, have highlighted the failure to understand the need for regular follow-up and the provision of ‘after-sales service’. The timing, indication and detailed operative findings at revision surgery have become important issues. Delays in revision surgery lead to progressive loss of bone and more complex technical problems. Regular follow-up after hip replacement with good-quality radiographs is essential. Cemented total hip arthroplasty continues to be an excellent method of treatment but, for young patients in particular, it is merely the beginning of the treatment. Key words: Charnley, long-term results, wear and loosening of the cup, strain shielding of the proximal femur, young patients.
2.1
Introduction
The success of total hip arthroplasty (THA) as a method of treating hip joints destroyed by arthritis has resulted in an increasing demand from an ageing population and has also extended the indications for this procedure. Consumer demand for new products stimulated the introduction of new materials and methods. Anecdotal, single-case, successes attract younger and less disabled patients. Immediate clinical success, short-term planning and pressure to satisfy the increasing demand, do not usually take into consideration the need for regular follow-up: the provision of ‘after-sales service’. 41
42
Orthopaedic bone cements
The availability of a successful ‘replacement’ procedure has also had a negative effect on the systematic study of underlying hip pathologies. It is not readily appreciated that clinical success is primarily due to pain relief. Pain relief is due to the natural, symptomatic joint being replaced with a mechanical neuropathic spacer; the whole construct functioning within a foreign body bursa. Wroblewski et al. (1999), in their long-term follow-up paper, stated that ‘clinical results do not reflect the mechanical state of the arthroplasty’ and that good-quality radiographs offer more information. Wroblewski et al. (2007a) also state that the activity level achieved after the operation is not a characteristic of a particular design, nor the method of component fixation, provided the fixation is secure; but it is a reflection of patient selection for the operation, patients with multiple disabilities never feature as anecdotal, single-case successes.
2.2
General principles
The method of treatment can be suitably reviewed under three main headings: design, materials and surgical technique.
2.2.1 Design The aim must be a set of components that allow a range of movements comparable with the natural joint, while ensuring stability without constraint. Any design must also take advantage of the best possible arrangements and combinations with respect to friction, frictional torque, wear and load transfer from the implant to the skeleton.
2.2.2 Materials Any combination of materials must be compatible with respect to each other and biocompatible with the tissues, not only in the solid but especially in their particulate form. They must also be fatigue and wear resistant.
2.2.3 Surgical technique Exposure must give an unimpeded, circumferential view of the acetabulum and the access to the medullary canal, in the area of the piriformis fossa, while preserving the integrity of the neighbouring structures as well as the all-important abductor muscle mass. Exposure of good-quality cancellous bone, pulse lavage, drying, containment, pressurisation of cement and correct timing of component insertion, are integral parts of the cementing technique.
Hip replacements
2.3
43
Bone cements
The characteristics of the various bone cements are well documented by Kühn (2000). Two aspects are of clinical importance: shrinkage on polymerisation and release of antibiotics. Shrinkage on polymerisation is the characteristic of the monomer. Our findings suggest that shrinkage occurs after setting/polymerisation has apparently taken place. The changes observed indicate that shrinkage would be away from both bone and the prosthesis. Under operative conditions the effect of the components acting as a ‘heat sink’ and intrusion into cancellous bone resisting the shrinkage, are yet to be quantified. Measured volume reduction is 25% (unpublished data), close to the 21% reported by Kühn (2000). Elution from polymerised cement is purely a surface phenomenon (Fig. 2.1). Wroblewski (1977), in his experimental evaluation, states ‘the volume released is proportional to the surface area of the cement mass’. Elution of gentamicin is neither continuous nor complete. Wroblewski et al. (1986) also noted that ‘on average 22% of the antibiotic is released within days of the operation’. Some of the remaining 78% will be released if exposure of the new cement surface occurs, as in cases of component loosening or cement fragmentation at a revision. This may affect samples taken for bacteriology.
2.1 A mixture of bone cement and gentian violet immersed in commercial bleach for 2 years. Note the thin outer bleached layer, some extensions of the process into the cement irregularities and the persistence of the dye within the cement mass.
44
Orthopaedic bone cements
2.4
Fixation of components with cement
Charnley (1963) in his lecture at the Société Internationale de Chirurgie Orthopédique et de Traumatologie (SICOT) said ‘The crux of the operation lies in the use of cement. By means of cement the load of the body weight is distributed over a large area of bone.’ This statement summarises succinctly all aspects of the technique. Charnley (1964) also emphasised that ‘Acrylic cement does not adhere to bone like glue, it merely forms an accurate cast of the interior of the bone so that load is transmitted evenly over all parts of the interface between cement and the cancellous bone’. This is the only method that offers immediately a ‘customised’ prosthesis in every case but also places the onus on the understanding and skill of the surgeon to achieve that aim. That skill is acquired by practice and is not a commodity that can be sold at a profit. This would probably explain the proliferation of ‘cementless’ designs. The terminology does not offer an indication of the method of component fixation except by stating what it is not, i.e. not with cement. It also hints that use of cement may be detrimental. Ultimately such designs become saleable commodities and take the responsibility for the quality of component fixation away from the surgeon and place it on the patient’s skeleton.
2.5
Long-term results
The Charnley low frictional torque arthroplasty (LFA) has now reached 45 years of continuous clinical application and must, therefore, be accepted as the gold standard (Wroblewski and Siney 1993).
2.5.1 Clinical results Using the Charnley LFA, pain relief is achieved and maintained; 96% of patients have either a totally pain-free hip or have no more than an occasional discomfort. Activity level, although improved and maintained, clearly depends on the underlying pathology and any factors that may become significant with time. Excellent, functional range of movements is achieved in 78% of patients and is maintained over the follow-up time.
2.5.2 Survivorship analysis Survivorship analysis used routinely in many hip registers continues to be the standard method of presenting long-term results with revision as the end point; each revision documented under a single indication (Soderman 2000). Our practice is to document all findings at revision (apart from infection which is a single finding). This gives a higher number of findings than
Hip replacements
45
Table 2.1 Survivorship analysis of 22 066 LFAs with a follow-up to 36 years
Reason for revision
Percentage success rate at 36 years’ follow-up
95% confidence limits
Infection Dislocation Fractured stem Loose stem Loose cup
91.4 98.0 88.6 72.5 48.6
100–67.9 100–85.9 100–62.4 100–39.2 79.1–18.0
hips revised and offers more valuable information (Wroblewski et al. 2007b). Survivorship analysis of the Charnley LFA, with a follow-up to 36 years is shown in Table 2.1.
2.6
Long-term problems
Two main long-term problems have been identified and addressed: (a) wear and loosening of the ultra-high molecular weight polyethylene (UHMWPE) cup and (b) strain shielding of the proximal femur. Charnley and Halley (1975) anticipated wear and loosening of the UHMWPE cup. They stated that ‘more than 5 mm wear might cause impingement of the neck of the prosthesis against the inner rim of the socket and cause loosening of the cement–bone bond in the acetabulum.’ The exponential correlation between wear and cup loosening has been documented in more recent studies (Wroblewski 1985a, Worblewski and Siney 1992, Worblewski et al. 2002). Issac et al. (1992) and Goldsmith et al. (2001), in their clinical and tribological studies, have identified factors affecting wear. The sequence of events of progressive penetration of the cup is restriction of movements, impingement of the neck of the stem on the rim of the cup and cup loosening. Reducing the diameter of the neck of the Charnley stem from 12.5 mm to 10 mm has delayed the impingement by the equivalent of 2 mm cup penetration. Clinical experience over the past 23 years (Wroblewski et al. 2004) supports the findings of the original theoretical model that used acrylic casts and shadowgraph techniques to measure real and radiographic wear (Wroblewski 1985b). Alumina ceramic/cross-linked polyethylene combinations, in the Charnley LFA, have now reached 20 years of clinical application (Wroblewski et al. 2005). The mean penetration rate is 0.02 mm/year with total penetration to date not exceeding 0.41 mm. Strain shielding of the proximal femur has been identified as being due to distal stem support not allowing proximal load transfer. The continuing developmental design of the stem, the C-Stem (DePuy International,
46
Orthopaedic bone cements
Leeds, UK) takes advantage of the common engineering principle: that of male (stem) and female (cement) tapers engaging under load. A number of factors essential for the concept to function have been detailed (Wroblewski et al. 2001). These are: a polished stem with a continuous triple taper; absence of distal stem support; and attention to the anatomical calcar. Clinical experience over the past 13 years has shown 100% survivorship – the end-point being stem loosening or revision – with 22% showing improvement in the radiographic appearance of the quality of the proximal femur.
2.7
Future trends
The cemented Charnley LFA has withstood the test of time. As with any THA, clinical results may not reflect the mechanical state, hence follow-up by good-quality radiographs is essential. Postal or telephone methods of collecting information are of limited value. Progressive loosening is an indication for revision, irrespective of the clinical result. This aspect must be understood and accepted by the patient, the surgeon and the system funding such a method of treatment. This must be agreed before the operation. Delays in revision surgery lead to progressive loss of bone stock and more complex technical problems to be tackled at revision. Long-term results are results in young patients. Increasing follow-up identifies ever-younger patients at the time of the operation. Now approaching 40 years’ follow-up, the mean age of patients at the time of the primary operation was 35 years. These are the results that have already been achieved with the Charnley LFA. Any newly proposed method or materials claiming to be suitable for young patients has the impossible task of providing data to support the statement that it will: accept patients with a mean age of 35 years and show a functioning arthroplasty when the patients reach the age of 75 years. This will not be achieved by a single generation of surgeons. Even then, continuity of design, materials and technique must be maintained. THA in general, and the Charnley LFA in particular, continue to be an excellent method of treatment, but for a young patient it merely heralds the beginning of treatment.
2.8
References
charnley j. 1963. Low friction arthroplasty of the hip in rheumatoid arthritis. SICOT Congress, Vienna, pp. 168–70. charnley, j. 1964. The bonding of prostheses to bone by cement. J Bone Joint Surg Br, 46, 518–29. charnley, j. & halley, d. k. 1975. Rate of wear in total hip replacement. Clin Orthop Relat Res, Oct (112), 170–9.
Hip replacements
47
goldsmith, a. a., dowson, d., wroblewski, b. m., siney, p. d., fleming, p. a. & lane, j. m. 2001. The effect of activity levels of total hip arthroplasty patients on socket penetration. J Arthroplasty, 16 (5), 620–7. isaac, g. h., wroblewski, b. m., atkinson, j. r. & dowson, d. 1992. A tribological study of retrieved hip prostheses. Clin Orthop Relat Res, Mar (276), 115–25. kühn k. d. 2000. Bone Cements. Up to Date Comparison of Physical and Chemical Properties of Commercial Materials. Springer, Berlin. soderman p. 2000. On the validity of the results from the Swedish National Total Hip Arthroplasty Register. Acta Orthop Scand, 71 (suppl 296), 4–33. wroblewski, b. m. 1977. Leaching out from acrylic bone cement. Experimental evaluation. Clin Orthop Relat Res, May (124), 311–2. wroblewski, b. m. 1985a. Charnley low-friction arthroplasty in patients under the age of 40 years. In Sevastik J. & Goldie, I. (eds), The Young Patient with Degenerative Hip Disease. Almquist and Wiksell, Stockholm, pp. 197–201. wroblewski, b. m. 1985b. Direction and rate of socket wear in Charnley low-friction arthroplasty. J Bone Joint Surg Br, 67 (5), 757–61. wroblewski, b. m., esser, m. & srigley, d. w. 1986. Release of gentamicin from bone cement. An ex-vivo study. Acta Orthop Scand, 57 (5), 413–4. wroblewski, b. m. & siney, p. d. 1992. Charnley low-friction arthroplasty in the young patient. Clin Orthop Relat Res, Dec (285), 45–7. wroblewski, b. m. & siney, p. d. 1993 Charnley low-friction arthroplasty of the hip. Long-term results. Clin Orthop Relat Res, Jul (292), 191–201. wroblewski, b. m., fleming, p. a. & siney, p. d. 1999. Charnley low-frictional torque arthroplasty of the hip. 20-to-30 year results. J Bone Joint Surg Br, 81 (3), 427–30. wroblewski, b. m., siney, p. d. & fleming, p. a. 2001. Triple taper polished cemented stem in total hip arthroplasty: rationale for the design, surgical technique, and 7 years of clinical experience. J Arthroplasty, 16 (Suppl 1), 37–41. wroblewski, b. m., siney, p. d. & fleming, p. a. 2002. Charnley low-frictional torque arthroplasty in patients under the age of 51 years. Follow-up to 33 years. J Bone Joint Surg Br, 84 (4), 540–3. wroblewski, b. m., siney, p. d. & fleming, p. a. 2004. Reduced diameter of the neck and its effect on the incidence of aseptic cup loosening in the Charnley LFA. J Bone Joint Surg Br, 87 (Suppl 1), 43. wroblewski, b. m., siney, p. d. & fleming, p. a. 2005. Low-friction arthroplasty of the hip using alumina ceramic and cross-linked polyethylene. A 17-year follow-up report. J Bone Joint Surg Br, 87 (9), 1220–1. wroblewski, b. m., siney, p. d. & fleming, p. a. 2007a. Charnley low-frictional torque arthroplasty in young rheumatoid and juvenile rheumatoid arthritis: 292 hips followed for an average of 15 years. Acta Orthop, 78 (2), 206–10. wroblewski, b. m., siney, p. d. & fleming, p. a. 2007b. Charnley low-friction arthroplasty: Survival patterns to 38 years. J Bone Joint Surg Br, 89, 1015–8.