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Press-fit hip arthroplasty: A European alternative
The Journal of Arthroplasty Vol. 17 No. 4 Suppl. 1 2002
Press-Fit Hip Arthroplasty A European Alternative Sergio Romagnoli, MD
Abstract: The Cement-Less Stem (CLS; Sulzer Medica, Baar, Switzerland) was borne out of the proximal fixation and mechanical stability through press-fit and secondary osseointegration theories. The biomechanical concept is characterized by the three-dimensional wedge-shaped taper, ribs in the proximal region, and the undersized tip of the stem. Histologic studies showed that the coarse-grained titanium alloy of the CLS provides primary stability. In a series of 300 consecutive CLS prostheses with a minimum follow-up of 10 years (range, 10 –16 years), femoral survivorship was 95% at 10 years and 90% at 14 years. Stem– bone fixation was stable, with bone ongrowth in 97% of patients, stable fibrous fixation in 1%, and unstable fibrous fixation in 2%. The CLS grit-blast, press-fit, collarless, tapered femoral component continues to perform well. Although continued surveillance is warranted, the good long-term results justify the continued use of the CLS for primary total hip arthroplasty. Key words: Total hip arthroplasty (THA), femoral stem, press-fit. Copyright 2002, Elsevier Science (USA). All rights reserved.
The term press-fit comes from mechanical engineering [1]. It means a mechanical joining of 2 parts based on contact pressure. Interference is the difference in diameter between the 2 parts and is the cause of deformation that occurs in each member. Friction between the 2 parts increases with contact pressure and serves to join the pieces together; a press-fit demands that relaxation (ie, viscoelastic behavior or creep) does not occur in either member. Viscoelastic behavior limits the effectiveness of the press-fit by relaxing the contact pressure at the interface [1]. It is possible to obtain press-fit between bone and metal, but it is arguable if this can be maintained
without some slippage of the components. In the case of a prosthesis inserted into the proximal femur, the elastic recoil of the bone about the prosthesis provides the initial large force per unit area. Viscoelastic relaxation decreases force on the prosthesis, and there is more relaxation of the less dense bone in the proximal region than there is of the more dense bone in the distal cortical region [2,3]. The living tissue, which leads to feedback and control of structure based on the mechanical environment, changes the structural properties of the bone and the press-fit [4]. The prosthesis is inserted into a healing fracture. There is an initial weakening of bone as substance is removed from the healing surface, and time is needed for adaptation of the new mechanical environment [3,4]. Injury to bone causes a regional acceleratory phenomenon that goes through the steps of activation, resorption, deposition, and quiescence, which lead to an initial weakening of the structure that is followed by a return of strength when new bone is laid down [5,6]. Such changes of remodeling continue, with each cycle at
From the Centro Chirurgia Protesica, Istituto Ortopedico “R. Galeazzi,” Milano, Italy. No benefits or funds were received in support of this study. Reprint requests: Sergio Romagnoli, MD, Istituto Ortopedico Galeazzi S.p.A., Via Riccardo Galeazzi 4, 20161 Milano, Italy. Copyright 2002, Elsevier Science (USA). All rights reserved. 0883-5403/02/1704-1030$35.00/0 doi:10.1054/arth.2002.32689
108
Press-Fit: A European Alternative • Sergio Romagnoli
a given site taking 3 to 4 months (the sigma period of bone) until the entire structure has adapted to the new strain environment around the prosthesis [7].
Cement-Less Stem Design The genesis for the development of the CementLess Stem (CLS) (Sulzer Medica, Baar, Switzerland) derived predominantly from the idea of proximal fixation of the stem and its mechanical stabilization through locking into the bone. This concept acquired a concrete expression through press-fit cementless total hip arthroplasty (THA) with unconventional design and through a new perspective with regard to surface configuration and choice of material. In 1980, Spotorno [8] developed a stem with a spiked surface in the proximal part. This prosthesis, made from a cobalt-chromium alloy, was crucial for the later design of the CLS. In 1983, longitudinally arranged ribs replaced the spiked structure of the surface, and titanium alloy, for its Young’s module characteristics, replaced cobalt-chromium [9]. The concept and design of the CLS have not changed to this day (Fig. 1). The ability to become integrated into bone is crucial for the success of uncemented implants. The biomechanical concept of the CLS is
Fig.1. CLS design: collarless three-dimensional tapered, straight, titanium alloy stem. The CLS is characterized by the three-dimensional wedge-shaped taper, the strongly accentuated ribs in the proximal region, and the all round conical form of the distal part of the stem.
109
characterized by the three-dimensional wedgeshaped taper, the strongly accentuated ribs in the proximal region, and the all round conical form of the distal part of the stem. The construction uses the adaptable cancellous bone trabecula for the primary fixation. The proximal fixation of the stem should guarantee that transmission of forces is retained within this area. The interposition of the cancellous structures reduces the danger of the bond between bone and prosthesis becoming stiff [3]. On introduction, the axial ribs exercise a cutting action and promote stability [4]. The shape of the distal part of the stem is kept rounded and relatively slender, and direct contact with the cortex is avoided where possible (Fig. 2). At implantation, the stem is stabilized by a press-fit. As viscoelastic relaxation occurs, the stem can subside minimally in the canal and find the level of stability by again obtaining a press-fit [10].
Indications The indications can be calculated on the basis of 4 key parameters: age, sex, degree of osteoporosis, and morphology of the femur. Each of these parameters is given a point score. The sum of the scores for all the parameters provides conclusive guidance on the choice of the implant. Concrete measures of the
110 The Journal of Arthroplasty Vol. 17 No. 4 Suppl. 1 June 2002
Fig. 2. Proximal fixation of the CLS (arrows). Distally, between lines, the shape of the stem is kept rounded and relatively slender, and direct contact with the cortex is avoided where possible.
degree of osteoporosis are provided by Singh’s index [11], which is based on studies of the trabecular structures of the femur. The morphologic-cortical index (MCI) [12] combined in 1 value 2 variables that do not always bear a reciprocal relationship: morphology (shape) of the femur and thickness of the cortex. The MCI consists of the ratio of the 2 sizes that can be measured on a standard radiograph of the femur in correct rotation: MCI ⫽ CD/AB, where CD is the distance between the outer limit of the lateral and medial cortex layer, measured on a level with the greatest prominence of the trochanter minor and vertically to the longitudinal axis of the femur, and AB is the diameter of the medullary canal, measured 7 cm distally to the CD line and vertically to the longitudinal axis of the femur. When MCI measures ⬎3.1, the femur is classified as trumpet-shaped; between 2.2 and 3, as intermediateshaped; and ⬍2.2, as cylindrical-shaped [8,13]. This measurement provides a reliable method of classification into 1 of 3 morphologic types: trumpetshaped, intermediate-shaped, or cylindrical-shaped. Indications for the CLS system depend on the bone remodeling capabilities and femoral shapes [10]. A cylindrical femur is one in which endosteal bone resorption is proceeding rapidly and the remaining bone is not likely to be able to sustain a press-fit. A trumpet-shaped femur generally has healthy bone, which is likely to maintain a press-fit and to remodel rapidly to a new strain environment. The
greater MCI value, the more likely a cementless prosthesis will lead to good results.
Results Histology After a successful operation, an artificial joint always remains a foreign body within the patient. To what extent and how quickly an implant becomes integrated is not just a question of primary stability. Important influential factors include the material and the surface structures of the prosthesis. Depending on their nature and composition, these factors can have a favorable effect on the formation of new bone in the direct vicinity of the prosthesis. In general, new bone grows preferentially on protruding parts, such as ribs or edges of the prosthesis, and on existing trabeculae, such as those altered by the surgery. The formation of new bone tissue in association with the CLS prosthesis was investigated thoroughly in histologic studies. A method was used whereby the various types of tissue and their condition could be made clearly visible and could be differentiated by staining. Histologic studies done by Schenk [14] showed that the bone in the proximal part of the ribs is not always able to bridge the gap between the tip of the rib and the main body of the shaft. The widening of
Press-Fit: A European Alternative • Sergio Romagnoli
111
Fig. 3. Histology of a retrieved stem. The bone in the proximal part of the ribs bridges the gap between the ribs and the main body of the shaft. The sharp edges provide stimulus for bone formation.
the rib grooves as a result of the sharpening of the edges provides more space and stimulus for the formation of bone. Two major findings emerge from this study. First, the coarse-grained titanium alloy is, to a great extent, osteophilic; this is shown by the fact that bone marrow or new bone can form directly on the surface of the CLS prosthesis (Fig. 3). Second, the shapes of the specific structures of the CLS prosthesis, which are adapted to the various areas of operation, provide primary stability. As a result, the optimal requirements for acceleration of osseointegration are met.
The average duration of long-term radiographic follow-up was 12.6 years (range, 10–16 years) (Fig. 4). Patients were evaluated clinically by the Harris hip score system [17]. At last examination, 5 hips were lost to follow-up, and 69 patients were dead. The femoral revision rate was 7% (2 aseptic loosening, 5 septic, 12 osteolysis). Femoral component survivorship was 95% at 10 years and 90% at 14
Clinical Results Long-term and intermediate-term clinical results of the CLS range from encouraging to outstanding [14,15]. The first 300 consecutive CLS prostheses (299 patients) implanted by Spotorno et al [8,13] were evaluated clinically and radiographically by Blaha et al [12] after 16 years of follow-up. The average age of the study cohort was 58 years (range, 21–77 years). Eight different types of acetabular components, including a cementless allpolyethylene socket in 80% of the cases, were used. Radiographic evaluation assessed Engh’s implant– bone femoral fixation score [16], implant– bone demarcation, and periprosthesis osteolysis.
Fig. 4. Radiographs of CLS and press-fit cup at 15 years of follow-up.
112 The Journal of Arthroplasty Vol. 17 No. 4 Suppl. 1 June 2002 years. The incidence of femoral periprosthetic osteolysis at last follow-up radiographic examination of ⱖ10 years was 47%, which included 12 hips (5%) with distal endosteal osteolysis. Femoral implant– bone fixation was observed to be stable, with bone ongrowth in 97% of the cases, stable fibrous fixation in 1%, and unstable fibrous fixation in 2%.
Conclusion Primary stability due to press-fit is converted to secondary, possibly definitive, fixation by means of direct bone fixation. The CLS grit-blast, press-fit, collarless, tapered femoral component continues to perform well clinically and radiographically up to 16 years of follow-up. This first generation cementless stem is still in use with virtually no changes in the design. The grit-blast femoral component offers a valid option for cementless hip replacement.
References 1. Shigley JE: Mechanical engineering design, 3rd ed. McGraw-Hill, New York, 1977 2. Lakes RS, Katz JL: Viscoelastic properties of wet cortical bone: II. relaxation mechanisms. J Biomech 12:679, 1979 3. Lakes RS, Katz JL, Sternstein SS: Viscoelastic properties of wet cortical bone: I. torsional and biaxial studies. J Biomech 12:657, 1979 4. Lanyon LE, Goodship AE, Pye CJ, McFie JH: Mechanically adaptive bone remodelling. J Biomech 15: 141, 1980 5. Frost HM: Bone remodelling and its relation to metabolic bone diseases. Charles C Thomas, Springfield, IL, 1973
6. Frost HM: The regional acceleratory phenomenon: a review. Henry Ford Med J 31:3, 1983 7. Frost HM: Bone remodelling dynamics. Charles C Thomas, Springfield, IL, 1963 8. Spotorno L, Romagnoli S, et al: The CLS system: theoretical concept and results. Acta Orthop Belg 59:144, 1993 9. Meunier A, Christel P, Sedel L, et al: The influence of the elasticity module of the femoral shaft and neck of a total hip prostheses on the distribution of stress in the femur. Int Orthop 14:67, 1990 10. Sabo D, Reiter A, Simank HG, et al: Periprosthetic mineralization around cementless total hip endoprosthesis: longitudinal study and cross-sectional study on titanium threaded acetabular cup and cementless Spotorno stem with DEXA. Calcif Tissue Int 62:177, 1998 11. Singh M, Nagrath AR, Maini PS: Changes in trabecular pattern of the upper end of the femur as an index of osteoporosis. J Bone Joint Surg Am 52:457, 1970 12. Blaha JD, Spotorno L, Romagnoli S: CLS press-fit total hip arthroplasty. Tech Orthop 6:80, 1991 13. Spotorno L, Romagnoli S, Ivaldo N: The cementless CLS Stem. In Kusswetter W (ed): Noncemented total hip replacement. Thieme, New York, 1991 14. Schenk RK, Wehrli U: Reaction of the bone to a cement-free SL femur revision prosthesis. Histologic find in an autopsy specimen 5 1/2 months after surgery. Orthopade 18:454-62, 1989 15. Spotorno L. An emerging gold standard for uncemented hip replacement? The CLS system a 16 year review. Int Orthop 24(Suppl):2000 16. Engh CA, Massin P, Suthers KE: Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components. Clin Orthop 257:107, 1990 17. Harris WH: Traumatic arthritis of the hip after dislocation and accetabular fractures. Treatment by mold arthroplasty. An end-results study using a new method of results evaluation. J Bone Joint Surg Am 51:737, 1969
Press-Fit Hip Arthroplasty A European Alternative Sergio Romagnoli, MD
Abstract: The Cement-Less Stem (CLS; Sulzer Medica, Baar, Switzerland) was borne out of the proximal fixation and mechanical stability through press-fit and secondary osseointegration theories. The biomechanical concept is characterized by the three-dimensional wedge-shaped taper, ribs in the proximal region, and the undersized tip of the stem. Histologic studies showed that the coarse-grained titanium alloy of the CLS provides primary stability. In a series of 300 consecutive CLS prostheses with a minimum follow-up of 10 years (range, 10 –16 years), femoral survivorship was 95% at 10 years and 90% at 14 years. Stem– bone fixation was stable, with bone ongrowth in 97% of patients, stable fibrous fixation in 1%, and unstable fibrous fixation in 2%. The CLS grit-blast, press-fit, collarless, tapered femoral component continues to perform well. Although continued surveillance is warranted, the good long-term results justify the continued use of the CLS for primary total hip arthroplasty. Key words: Total hip arthroplasty (THA), femoral stem, press-fit. Copyright 2002, Elsevier Science (USA). All rights reserved.
The term press-fit comes from mechanical engineering [1]. It means a mechanical joining of 2 parts based on contact pressure. Interference is the difference in diameter between the 2 parts and is the cause of deformation that occurs in each member. Friction between the 2 parts increases with contact pressure and serves to join the pieces together; a press-fit demands that relaxation (ie, viscoelastic behavior or creep) does not occur in either member. Viscoelastic behavior limits the effectiveness of the press-fit by relaxing the contact pressure at the interface [1]. It is possible to obtain press-fit between bone and metal, but it is arguable if this can be maintained
without some slippage of the components. In the case of a prosthesis inserted into the proximal femur, the elastic recoil of the bone about the prosthesis provides the initial large force per unit area. Viscoelastic relaxation decreases force on the prosthesis, and there is more relaxation of the less dense bone in the proximal region than there is of the more dense bone in the distal cortical region [2,3]. The living tissue, which leads to feedback and control of structure based on the mechanical environment, changes the structural properties of the bone and the press-fit [4]. The prosthesis is inserted into a healing fracture. There is an initial weakening of bone as substance is removed from the healing surface, and time is needed for adaptation of the new mechanical environment [3,4]. Injury to bone causes a regional acceleratory phenomenon that goes through the steps of activation, resorption, deposition, and quiescence, which lead to an initial weakening of the structure that is followed by a return of strength when new bone is laid down [5,6]. Such changes of remodeling continue, with each cycle at
From the Centro Chirurgia Protesica, Istituto Ortopedico “R. Galeazzi,” Milano, Italy. No benefits or funds were received in support of this study. Reprint requests: Sergio Romagnoli, MD, Istituto Ortopedico Galeazzi S.p.A., Via Riccardo Galeazzi 4, 20161 Milano, Italy. Copyright 2002, Elsevier Science (USA). All rights reserved. 0883-5403/02/1704-1030$35.00/0 doi:10.1054/arth.2002.32689
108
Press-Fit: A European Alternative • Sergio Romagnoli
a given site taking 3 to 4 months (the sigma period of bone) until the entire structure has adapted to the new strain environment around the prosthesis [7].
Cement-Less Stem Design The genesis for the development of the CementLess Stem (CLS) (Sulzer Medica, Baar, Switzerland) derived predominantly from the idea of proximal fixation of the stem and its mechanical stabilization through locking into the bone. This concept acquired a concrete expression through press-fit cementless total hip arthroplasty (THA) with unconventional design and through a new perspective with regard to surface configuration and choice of material. In 1980, Spotorno [8] developed a stem with a spiked surface in the proximal part. This prosthesis, made from a cobalt-chromium alloy, was crucial for the later design of the CLS. In 1983, longitudinally arranged ribs replaced the spiked structure of the surface, and titanium alloy, for its Young’s module characteristics, replaced cobalt-chromium [9]. The concept and design of the CLS have not changed to this day (Fig. 1). The ability to become integrated into bone is crucial for the success of uncemented implants. The biomechanical concept of the CLS is
Fig.1. CLS design: collarless three-dimensional tapered, straight, titanium alloy stem. The CLS is characterized by the three-dimensional wedge-shaped taper, the strongly accentuated ribs in the proximal region, and the all round conical form of the distal part of the stem.
109
characterized by the three-dimensional wedgeshaped taper, the strongly accentuated ribs in the proximal region, and the all round conical form of the distal part of the stem. The construction uses the adaptable cancellous bone trabecula for the primary fixation. The proximal fixation of the stem should guarantee that transmission of forces is retained within this area. The interposition of the cancellous structures reduces the danger of the bond between bone and prosthesis becoming stiff [3]. On introduction, the axial ribs exercise a cutting action and promote stability [4]. The shape of the distal part of the stem is kept rounded and relatively slender, and direct contact with the cortex is avoided where possible (Fig. 2). At implantation, the stem is stabilized by a press-fit. As viscoelastic relaxation occurs, the stem can subside minimally in the canal and find the level of stability by again obtaining a press-fit [10].
Indications The indications can be calculated on the basis of 4 key parameters: age, sex, degree of osteoporosis, and morphology of the femur. Each of these parameters is given a point score. The sum of the scores for all the parameters provides conclusive guidance on the choice of the implant. Concrete measures of the
110 The Journal of Arthroplasty Vol. 17 No. 4 Suppl. 1 June 2002
Fig. 2. Proximal fixation of the CLS (arrows). Distally, between lines, the shape of the stem is kept rounded and relatively slender, and direct contact with the cortex is avoided where possible.
degree of osteoporosis are provided by Singh’s index [11], which is based on studies of the trabecular structures of the femur. The morphologic-cortical index (MCI) [12] combined in 1 value 2 variables that do not always bear a reciprocal relationship: morphology (shape) of the femur and thickness of the cortex. The MCI consists of the ratio of the 2 sizes that can be measured on a standard radiograph of the femur in correct rotation: MCI ⫽ CD/AB, where CD is the distance between the outer limit of the lateral and medial cortex layer, measured on a level with the greatest prominence of the trochanter minor and vertically to the longitudinal axis of the femur, and AB is the diameter of the medullary canal, measured 7 cm distally to the CD line and vertically to the longitudinal axis of the femur. When MCI measures ⬎3.1, the femur is classified as trumpet-shaped; between 2.2 and 3, as intermediateshaped; and ⬍2.2, as cylindrical-shaped [8,13]. This measurement provides a reliable method of classification into 1 of 3 morphologic types: trumpetshaped, intermediate-shaped, or cylindrical-shaped. Indications for the CLS system depend on the bone remodeling capabilities and femoral shapes [10]. A cylindrical femur is one in which endosteal bone resorption is proceeding rapidly and the remaining bone is not likely to be able to sustain a press-fit. A trumpet-shaped femur generally has healthy bone, which is likely to maintain a press-fit and to remodel rapidly to a new strain environment. The
greater MCI value, the more likely a cementless prosthesis will lead to good results.
Results Histology After a successful operation, an artificial joint always remains a foreign body within the patient. To what extent and how quickly an implant becomes integrated is not just a question of primary stability. Important influential factors include the material and the surface structures of the prosthesis. Depending on their nature and composition, these factors can have a favorable effect on the formation of new bone in the direct vicinity of the prosthesis. In general, new bone grows preferentially on protruding parts, such as ribs or edges of the prosthesis, and on existing trabeculae, such as those altered by the surgery. The formation of new bone tissue in association with the CLS prosthesis was investigated thoroughly in histologic studies. A method was used whereby the various types of tissue and their condition could be made clearly visible and could be differentiated by staining. Histologic studies done by Schenk [14] showed that the bone in the proximal part of the ribs is not always able to bridge the gap between the tip of the rib and the main body of the shaft. The widening of
Press-Fit: A European Alternative • Sergio Romagnoli
111
Fig. 3. Histology of a retrieved stem. The bone in the proximal part of the ribs bridges the gap between the ribs and the main body of the shaft. The sharp edges provide stimulus for bone formation.
the rib grooves as a result of the sharpening of the edges provides more space and stimulus for the formation of bone. Two major findings emerge from this study. First, the coarse-grained titanium alloy is, to a great extent, osteophilic; this is shown by the fact that bone marrow or new bone can form directly on the surface of the CLS prosthesis (Fig. 3). Second, the shapes of the specific structures of the CLS prosthesis, which are adapted to the various areas of operation, provide primary stability. As a result, the optimal requirements for acceleration of osseointegration are met.
The average duration of long-term radiographic follow-up was 12.6 years (range, 10–16 years) (Fig. 4). Patients were evaluated clinically by the Harris hip score system [17]. At last examination, 5 hips were lost to follow-up, and 69 patients were dead. The femoral revision rate was 7% (2 aseptic loosening, 5 septic, 12 osteolysis). Femoral component survivorship was 95% at 10 years and 90% at 14
Clinical Results Long-term and intermediate-term clinical results of the CLS range from encouraging to outstanding [14,15]. The first 300 consecutive CLS prostheses (299 patients) implanted by Spotorno et al [8,13] were evaluated clinically and radiographically by Blaha et al [12] after 16 years of follow-up. The average age of the study cohort was 58 years (range, 21–77 years). Eight different types of acetabular components, including a cementless allpolyethylene socket in 80% of the cases, were used. Radiographic evaluation assessed Engh’s implant– bone femoral fixation score [16], implant– bone demarcation, and periprosthesis osteolysis.
Fig. 4. Radiographs of CLS and press-fit cup at 15 years of follow-up.
112 The Journal of Arthroplasty Vol. 17 No. 4 Suppl. 1 June 2002 years. The incidence of femoral periprosthetic osteolysis at last follow-up radiographic examination of ⱖ10 years was 47%, which included 12 hips (5%) with distal endosteal osteolysis. Femoral implant– bone fixation was observed to be stable, with bone ongrowth in 97% of the cases, stable fibrous fixation in 1%, and unstable fibrous fixation in 2%.
Conclusion Primary stability due to press-fit is converted to secondary, possibly definitive, fixation by means of direct bone fixation. The CLS grit-blast, press-fit, collarless, tapered femoral component continues to perform well clinically and radiographically up to 16 years of follow-up. This first generation cementless stem is still in use with virtually no changes in the design. The grit-blast femoral component offers a valid option for cementless hip replacement.
References 1. Shigley JE: Mechanical engineering design, 3rd ed. McGraw-Hill, New York, 1977 2. Lakes RS, Katz JL: Viscoelastic properties of wet cortical bone: II. relaxation mechanisms. J Biomech 12:679, 1979 3. Lakes RS, Katz JL, Sternstein SS: Viscoelastic properties of wet cortical bone: I. torsional and biaxial studies. J Biomech 12:657, 1979 4. Lanyon LE, Goodship AE, Pye CJ, McFie JH: Mechanically adaptive bone remodelling. J Biomech 15: 141, 1980 5. Frost HM: Bone remodelling and its relation to metabolic bone diseases. Charles C Thomas, Springfield, IL, 1973
6. Frost HM: The regional acceleratory phenomenon: a review. Henry Ford Med J 31:3, 1983 7. Frost HM: Bone remodelling dynamics. Charles C Thomas, Springfield, IL, 1963 8. Spotorno L, Romagnoli S, et al: The CLS system: theoretical concept and results. Acta Orthop Belg 59:144, 1993 9. Meunier A, Christel P, Sedel L, et al: The influence of the elasticity module of the femoral shaft and neck of a total hip prostheses on the distribution of stress in the femur. Int Orthop 14:67, 1990 10. Sabo D, Reiter A, Simank HG, et al: Periprosthetic mineralization around cementless total hip endoprosthesis: longitudinal study and cross-sectional study on titanium threaded acetabular cup and cementless Spotorno stem with DEXA. Calcif Tissue Int 62:177, 1998 11. Singh M, Nagrath AR, Maini PS: Changes in trabecular pattern of the upper end of the femur as an index of osteoporosis. J Bone Joint Surg Am 52:457, 1970 12. Blaha JD, Spotorno L, Romagnoli S: CLS press-fit total hip arthroplasty. Tech Orthop 6:80, 1991 13. Spotorno L, Romagnoli S, Ivaldo N: The cementless CLS Stem. In Kusswetter W (ed): Noncemented total hip replacement. Thieme, New York, 1991 14. Schenk RK, Wehrli U: Reaction of the bone to a cement-free SL femur revision prosthesis. Histologic find in an autopsy specimen 5 1/2 months after surgery. Orthopade 18:454-62, 1989 15. Spotorno L. An emerging gold standard for uncemented hip replacement? The CLS system a 16 year review. Int Orthop 24(Suppl):2000 16. Engh CA, Massin P, Suthers KE: Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components. Clin Orthop 257:107, 1990 17. Harris WH: Traumatic arthritis of the hip after dislocation and accetabular fractures. Treatment by mold arthroplasty. An end-results study using a new method of results evaluation. J Bone Joint Surg Am 51:737, 1969