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Changes in bone density after cemented total knee arthroplasty
The Journal of Arthroplasty Vol. 16 No. 1 2001
Changes in Bone Density After Cemented Total Knee Arthroplasty Influence of Stem Design Jess H. Lonner, MD, Michael Klotz, MD, Craig Levitz, MD, and Paul A. Lotke, MD
Abstract: Total knee arthroplasty has shown excellent survivorship in short-term and intermediate-term studies. With longer follow-up, however, aseptic loosening becomes an increasing cause of failure. Dual-energy x-ray absorptiometry scanning has shown that stress shielding occurs from altered mechanical loading. The purpose of this study is to determine if tibial stem design affects bone density in the longterm. Bone densities in the proximal tibia with and without cemented stems were compared at an average of 94 months after surgery. The bone quality under the Miller-Galante I prosthesis, which has 4 0.5-cm pegs, was compared with the bone quality under a Press-Fit Condylar prosthesis with a single 4-cm stem. Each group was also compared with the unoperated contralateral tibia. Results showed that there is a significantly reduced density of bone in the tibial metaphysis in the cemented stemmed group but not in the pegged group. There were no changes distally in the diaphyseal bone. This study supports the contention that the use of a cemented stem reduces proximal stresses and may result in proximal bone resorption. Although the use of a stem provides excellent resistance to lift-off and shear, it comes at a price. The proximal resorption may contribute to the persistence of tibial component loosening as a primary threat to survivorship. This bone loss may complicate revision surgery. Consideration should be given to using shorter tibial stems, less cement, or alternative designs that avoid long-stem fixation. Key words: total knee arthroplasty, dual-energy x-ray absorptiometry, stress shielding.
Total knee arthroplasty (TKA) has proved effective for the treatment of advanced arthritis of the knee, with excellent performance beyond 10 years [1,2]. Although many of the failures result from polyethylene wear, some clinical failures result from col-
lapse of the trabecular bone in the tibial metaphysis. To address this problem, metal backing of the tibial component was introduced to distribute the stresses more evenly across the tibia [3]. Despite these efforts, aseptic loosening of the tibial component is a troublesome and possibly an increasing cause of failure [4,5]. Reports of total hip arthroplasty (THA) have focused on changes in the quality of the proximal femoral bone [6 –11]. Some degree of bone loss has been noted with prosthetic implantation, suggesting that this loss may reflect the bone response to altered mechanical loading conditions around the femoral stem. Only a few studies have documented the effects of TKAs on bone [8,12–14]. A report from this institution [8] revealed an average 36.4%
From the Penn Orthopaedic Institute, Department of Orthopaedic Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Submitted January 24, 2000; accepted March 21, 2000. No benefits or funds were received in support of this study. Reprint requests: Jess H. Lonner, MD, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104. Copyright © 2001 by Churchill Livingstone威 0883-5403/01/1601-0018$10.00/0 doi:10.1054/arth.2001.16486
107
108 The Journal of Arthroplasty Vol. 16 No. 1 January 2001 proximal tibial bone loss 8 years after TKA. This finding suggests that weakening of the proximal tibial bone also may occur after TKA and contribute to aseptic loosening of the tibial component. The purpose of this study was to determine the effect of stem design on bone density in the proximal tibia using dual-energy x-ray absorptiometry (DEXA) scan analysis in long-term well-functioning TKAs in elderly women. It is unknown whether tibial baseplates of differing geometries with and without tibial stems can modulate the effect seen in the earlier study.
Materials and Methods
Table 1. Summary of Results
Age (y) Height (inches) Weight (lb) Follow-up (mo) Knee Society score Clinical Functional Range of motion (°) Flexion preoperative Flexion postoperative Tibiofemoral alignment (°) Preoperative Postoperative
Miller-Galante
Press-Fit Condylar
71.1 (65–86) 63.8 (62–66) 163 (143–180) 95 (81–98)
73.6 (67–78) 62.5 (61–64) 160 (138–178) 94 (84–101)
93 (88–97) 73 (55–90)
88 (67–98) 67 (45–90)
110 (95–120) 105 (90–115)
110 (100–130) 110 (95–115)
180 (176–184) 174 (173–175)
174 (176–180) 174 (173–175)
NOTE. Values are averages with ranges in parentheses.
The study group comprised 12 asymptomatic healthy white women who underwent TKA for unilateral osteoarthrosis between January 1987 and April 1989 and had a minimal follow-up of 7 years. Patients with contralateral knee pathology or surgery; patients with medical conditions known to alter bone density, such as rheumatoid arthritis; and patients on long-term steroid therapy were excluded. Two implant types with distinct stem designs were used: the Miller-Galante (MG) TKA, (Zimmer, Warsaw IN), with a tibial plate with 4 short (0.5cm) cemented fixation pegs (6 patients), and the Press-Fit Condylar (PFC) TKA (Johnson & Johnson, Rayham, MA), with a 4-cm cemented central stem (6 patients). All perioperative and operative techniques were similar and contemporaneous. All components were cemented in place, including the stems. Postoperative Knee Society knee function and radiograph scores were obtained for all patients. All patients were asymptomatic, and standing radiographs on long plates showed well-positioned prostheses without evidence of loosening or wear. A bilateral DEXA scan of the knees was performed. Bone density was measured at 4 standardized locations (Fig. 1) as described by Levitz et
Fig. 1. Bone density was measured in 4 different areas in the normal control and beneath each prosthesis.
al [8]. For each patient, the ratio of bone density for the operated knee to the corresponding location on the unoperated knee was determined. Statistical analysis was completed with the SAS, including 1-way analysis of variance summary of fit, t-test, and analysis of variance (P ⬍ .05).
Results The average height, weight, and duration of follow-up were similar in both groups (Table 1). The interval from surgery to DEXA scan analysis averaged 95 months in the MG group and 94 months in the PFC group. Knee Society clinical and function scores were 93 and 73 in the MG group and 88 and 67 in the PFC group. These differences were not statistically significant. The preoperative tibiofemoral alignment was similar in the 2 groups (Table 1). Average postoperative alignment was 174° because perfect alignment was required for entry into the study. Two patients used a cane in each group. The bone densities under the medial and lateral plateaus in the stemmed PFC group were significantly reduced as compared with the unstemmed MG group (P ⬍ .001). Distally the diaphyseal densities were not significantly different in each group. Bone density ratios, comparing the tibial bone beneath the medial and lateral plateaus with the nonoperated contralateral tibia were significantly different for each group (P ⬍ .0016). There were no significant differences in the ratios of bone density in the 2 diaphyseal measurements (Table 2).
Bone Density After Cemented TKA • Lonner et al.
109
Table 2. Dual-Energy X-Ray Absorptiometry Scan Results Medial Plateau Bone density (g/cm2) MG 0.794 ⫾ 0.121 PFC 0.225 ⫾ 0.097 Contra 0.612 ⫾ 0.072 P value ⬍.001 Ratio to contralateral tibia MG 1.03 ⫾ 0.10 PFC 0.30 ⫾ 0.10 P value .004
Lateral Plateau
Proximal Diaphysis
Distal Diaphysis
0.809 ⫾ 0.162 0.346 ⫾ 0.113 0.670 ⫾ 0.080 ⬍.001
0.793 ⫾ 0.157 0.542 ⫾ 0.139 0.733 ⫾ 0.094 NS
1.06 ⫾ 0.206 0.819 ⫾ 0.093 0.988 ⫾ 0.111 NS
1.27 ⫾ 0.14 0.44 ⫾ 0.14 .0023
0.93 ⫾ 0.13 0.80 ⫾ 0.13 NS
1.0 ⫾ 0.05 0.88 ⫾ 0.05 NS
MG, Miller-Galante; PFC, Press-Fit Condylar; Contra, contralateral tibia.
Discussion Stress shielding and bone resorption after TKA and THA have become a clinical problem and may contribute to prosthesis failure. Bone loss resulting from prosthetic implantation may compromise the stability of the implant itself and make revision surgery more difficult. The results of this study suggest that there is a greater degree of bone loss associated with the use of a tibial component fixed by a cemented stem as compared with an unstemmed design. This finding may have significant implications for the longevity of present TKA designs and for the design of future implants. DEXA scanning has become a useful tool in the determination of bone densities in many locations. With newer computer software packages, the scans are more accurate in the regions around metallic implants. In the proximal femur, accuracy of the DEXA scan is reported to be within 1% for cadaveric specimens and to range from 2% to 6.8% for in vivo measurements [15,16]. Most of the variability in clinical studies is secondary to minor variations in rotation between measurements. Bone density measurements have been shown to correlate closely with the mechanical properties of bone [17– 19], suggesting that bone loss identified by DEXA scan can be a threat to implant stability. At present few data are available on the DEXA measurements around TKAs in vivo, although the data appear similar to the increasing experience in THAs. Robertson et al [28] studied bone loss in a cadaveric distal femur after TKA. Sections of bone measuring 3 mm were removed sequentially, and standard radiographs and DEXA scans were obtained. DEXA scanning was shown to be significantly more accurate than visual interpretation or computer analysis of the radiographs, with a coefficient of variation of 1.2%. The DEXA bone measurements were highly correlated with the calcium
ash content of the bone, with a correlation coefficient of 1. Many TKA designs have used a tibial stem to augment fixation. Initial experience with cemented, all-polyethylene stemmed components showed that although the stems often remained firmly fixed in place, polyethylene deformation caused by eccentric loading led to mechanical failure of the bone in the medial or lateral plateau [21]. Analysis of these tibial components [3,22,23] revealed that there was little axial load bearing through the polyethylene stems, with all stresses being transferred to the proximal bone. As a result of these observations, to protect the proximal bone from failure, metal backing was added to the tibial component. Finite element analysis [3,6] reveals that the addition of metal reduces the overall stresses on the cancellous surface and results in a more even distribution of stresses, especially in eccentric varus or valgus loading conditions. Comparing various designs for fixation of the plate to the bone, Walker et al [24] found that a centrally placed metal stem was found to enhance further resistance to lift-off in eccentric loading as well as improve resistance to shear stresses and was thought to represent the most secure fixation. The axial load bearing by the stem was found to be 28%. Bartel et al [3] concluded that the stem would reduce the overall load carried by the proximal cancellous bone and that the metal tray would improve the distribution of the remaining stresses. Brooks et al [25] and Bourne and Finlay [21] have used arrays of strain gauges to quantitate the stress changes associated with stemmed tibial components, concluding that there is a marked reduction in stresses measured in all locations proximal to the stem. The latter authors speculated that this stress shielding may jeopardize long-term fixation of the tibial component as a result of bone resorption. Whiteside and Pafford [26] evaluated the
110 The Journal of Arthroplasty Vol. 16 No. 1 January 2001 load-bearing capacity of 2 stemmed implants, 1 cemented and 1 press-fit, and found proximal stress shielding of an equal magnitude with each design. Long-term changes in bone density of the distal femur associated with TKA have been documented. Petersen et al [13] found a 36% decrease in bone density behind the anterior femoral flange at 2 years, with a 22% increase in density proximal to the fixation pegs, findings that they attributed to altered mechanical loading conditions. Reports of short-term tibial bone density changes [12,14] revealed little or no change 3 years after TKA [8]. This study compared 2 distinct designs from different manufacturers. The presence or absence of a central tibial stem may not be the sole influence on proximal tibial bone density. Other factors, such as tray thickness and metallurgy, may play a role. The 2 implants tested in this series were both titanium, but platform thickness differed. The MG tray is 3.6 mm thick, and the PFC tray is 2.7 mm thick. Based on this information, one might surmise that the MG tray would be stiffer and stress shield more, if all other variables were controlled perfectly, but we are not aware of tested differences in the stiffness of the baseplates. In addition to these factors and stem geometry, the cement mantle geometry may affect the stress distribution to the underlying bone. This factor was not addressed in this study. The apparent loss of bone density that may result from osteolysis and reaction to particulate debris should be considered. In this series, all patients were asymptomatic, without effusion or evidence of polyethylene wear on standing radiographs. This study supports the contention that use of a cemented stem reduces proximal stresses and may result in proximal bone resorption. This proximal bone loss is not seen in the unstemmed design tested. Although the use of a central stem may enhance component stability, with resistence to lift-off and shear, it comes at a price. The proximal resorption may contribute to the persistence of tibial component loosening as a primary threat to TKA survivorship. This bone loss may complicate revision surgery and can compromise the quality and outcome of a TKA revision. It is unproven that in a cemented tibial component, the stability to lift-off and toggle provided by a central stem is any better than that provided by small pegs [29,30]. In a study by Lonner et al [29], comparing the stability of 4 cemented tibial components with different keel designs, no significant difference was observed in component lift-off or subsidence with application of physiologic cyclic eccentric loads. Two of the implants tested in that earlier study were studied clinically in the present study. In another study,
Stern et al [30] analyzed the micromotion and tilt observed in a single implant design with stem extensions of varying lengths. The authors concluded that in cemented tibial components, in contrast to cementless implants, stem extensions, in the presence of reasonable metaphyseal bone stock, do not enhance initial stability and are unnecessary for most routine primary cemented TKAs [30]. These results are not analogous to the results observed in cementless implants. Implant stability, whether cemented or cementless, is crucial to prevent lift-off, subsidence, and loosening. In the future, consideration should be given to shorter tibial stems or alternate designs that minimize stem fixation for primary cemented TKA.
References 1. Colizza WA, Insall JN, Scuderi GR: The posterior stabilized total knee prosthesis: assessment of polyethylene damage and osteolysis after ten year minimum follow up. J Bone Joint Surg Am 74:1713, 1995 2. Stern SH, Insall JN: Posterior stabilized prosthesis: results after follow-up of nine to twelve years. J Bone Joint Surg Am 74:980, 1992 3. Bartel DH, Burstein AH, Santavicca EA, et al: Performance of the tibial component in total knee replacement. J Bone Joint Surg Am 64:1026, 1982 4. Malkani AL, Rand JA, Bryan RS, et al: Total knee arthroplasty with the kinematic condylar prosthesis: a ten-year follow-up study. J Bone Joint Surg Am 77:423, 1995 5. Moran CG, Pinder IM, Lees TA, et al: Survivorship analysis of the uncemented porous-coated anatomic knee replacement. J Bone Joint Surg Am 73:848, 1991 6. Cohen B, Rushton N: Bone remodelling in the proximal femur after Charnley total hip arthroplasty. J Bone Joint Surg Br 77:815, 1995 7. Hughes SS, Furia JP, Smith P, et al: Atrophy of the proximal part of the femur after total hip arthroplasty without cement: a quantitative comparison of cobaltchromium and titanium femoral stems with use of dual x-ray absorptiometry. J Bone Joint Surg Am 77:231, 1995 8. Levitz CL, Lotke PA, Karp JS: Long-term changes in bone mineral density following total knee replacement. Clin Orthop 321:68, 1995 9. McGovern TF, Engh CA, Zettl-Schaffer K, et al: Cortical bone density of the proximal femur following cementless total hip arthroplasty. Clin Orthop 306: 145, 1994 10. Niinimaki T, Jalovaara P: Bone loss from the proximal femur after arthroplasty with an isoelastic femoral stem: BMD measurements in 25 patients after 9 years. Acta Orthop Scand 66:347, 1995 11. Pritchett JW: Femoral bone loss following hip re-
Bone Density After Cemented TKA • Lonner et al.
12.
13.
14.
15.
16.
17.
18.
19.
20.
placement: a comparative study. Clin Orthop 314: 156, 1995 Bohr HH, Lund B: Bone mineral density of the proximal tibia following uncemented arthroplasty. J Arthroplasty 2:309, 1987 Petersen MM, Olsen C, Lauritzen JB, et al: Changes in bone mineral density of the distal femur following uncemented total knee arthroplasty. J Arthroplasty 10:7, 1995 Seitz P, Ruegsegger P, Gschwend N, et al: Changes in local bone density after total knee replacement. J Bone Joint Surg Br 69:407, 1987 Cohen B, Rushton N: Accuracy of DEXA measurement of bone mineral density after total hip arthroplasty. J Bone Joint Surg Br 77:479, 1995 Trevisan C, Bigoni M, Cherubini R, et al: Dual x-ray absorptiometry for the evaluation of bone density from the proximal femur after total hip arthroplasty: analysis protocols and reproducibility. Calcif Tissue Int 53:158, 1993 Houde J, Marchetti M, Duquette J, et al: Correlation of bone mineral density and femoral neck hardness in bovine and human samples. Calif Tissue Int 57: 201, 1995 Mazess RB, Barden HS: Measurement of bone by dual photon absorptiometry and dual energy x-ray absorptiometry. Ann Chir Gynaecol 77:197, 1988 Nelson D, Feingold M, Mascha E, et al: Comparison of single photon and dual energy x-ray absorptiometry. Bone Miner 18:77, 1992 Ritter MA, Herbst SA, Keating EM, et al: Long-term survival analysis of a posterior cruciate-retaining total condylar total knee arthroplasty. Clin Orthop 309: 136, 1994
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21. Bourne RB, Finlay JB: The influence of tibial component intramedullary stems and implant-cortex contact on the strain distribution of the proximal tibia following total knee arthroplasty. Clin Orthop 208:95, 1986 22. Lewis JL, Askew MJ, Jaycox DP: A comparative evaluation of tibial component designs of total knee prostheses. J Bone Joint Surg Am 64:129, 1982 23. Reilly D, Walker PS, Ben-Dov M, et al: Effects of tibial components on load transfer in the upper tibia. Clin Orthop 165:273, 1982 24. Walker PS, Hsu H, Zimmerman RA: A comparative study of uncemented tibial components. J Arthroplasty 5:245, 1990 25. Brooks PJ, Walker PS, Scott RD: Tibial component fixation in deficient tibial bone stock. Clin Orthop 184:302, 1984 26. Whiteside LA, Pafford J: Load transfer characteristics of a noncemented total knee arthroplasty. Clin Orthop 239:168, 1989 27. Ranawat CS, Flynn WF Jr, Saddler S, et al: Long-term results of the total condylar knee arthroplasty: a 15 year survivorship study. Clin Orthop 286:94, 1993 28. Robertson DD, Mintzer CM, Weissman BN, et al: Distal loss of femoral bone following total knee arthroplasty. J Bone Joint Surg Am 76:66, 1994 29. Lonner JH, Pearlman C, Kummer FJ, et al: The effect of keel design on tibial component stability in total knee arthroplasty. Proceedings of the Scientific Meeting of the Knee Society, San Francisco, February 1997 30. Stern SH, Wills RD, Gilbert JL: The effect of tibial stem design on component micromotion in knee arthroplasty. Clin Orthop 345:44, 1997
Changes in Bone Density After Cemented Total Knee Arthroplasty Influence of Stem Design Jess H. Lonner, MD, Michael Klotz, MD, Craig Levitz, MD, and Paul A. Lotke, MD
Abstract: Total knee arthroplasty has shown excellent survivorship in short-term and intermediate-term studies. With longer follow-up, however, aseptic loosening becomes an increasing cause of failure. Dual-energy x-ray absorptiometry scanning has shown that stress shielding occurs from altered mechanical loading. The purpose of this study is to determine if tibial stem design affects bone density in the longterm. Bone densities in the proximal tibia with and without cemented stems were compared at an average of 94 months after surgery. The bone quality under the Miller-Galante I prosthesis, which has 4 0.5-cm pegs, was compared with the bone quality under a Press-Fit Condylar prosthesis with a single 4-cm stem. Each group was also compared with the unoperated contralateral tibia. Results showed that there is a significantly reduced density of bone in the tibial metaphysis in the cemented stemmed group but not in the pegged group. There were no changes distally in the diaphyseal bone. This study supports the contention that the use of a cemented stem reduces proximal stresses and may result in proximal bone resorption. Although the use of a stem provides excellent resistance to lift-off and shear, it comes at a price. The proximal resorption may contribute to the persistence of tibial component loosening as a primary threat to survivorship. This bone loss may complicate revision surgery. Consideration should be given to using shorter tibial stems, less cement, or alternative designs that avoid long-stem fixation. Key words: total knee arthroplasty, dual-energy x-ray absorptiometry, stress shielding.
Total knee arthroplasty (TKA) has proved effective for the treatment of advanced arthritis of the knee, with excellent performance beyond 10 years [1,2]. Although many of the failures result from polyethylene wear, some clinical failures result from col-
lapse of the trabecular bone in the tibial metaphysis. To address this problem, metal backing of the tibial component was introduced to distribute the stresses more evenly across the tibia [3]. Despite these efforts, aseptic loosening of the tibial component is a troublesome and possibly an increasing cause of failure [4,5]. Reports of total hip arthroplasty (THA) have focused on changes in the quality of the proximal femoral bone [6 –11]. Some degree of bone loss has been noted with prosthetic implantation, suggesting that this loss may reflect the bone response to altered mechanical loading conditions around the femoral stem. Only a few studies have documented the effects of TKAs on bone [8,12–14]. A report from this institution [8] revealed an average 36.4%
From the Penn Orthopaedic Institute, Department of Orthopaedic Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Submitted January 24, 2000; accepted March 21, 2000. No benefits or funds were received in support of this study. Reprint requests: Jess H. Lonner, MD, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104. Copyright © 2001 by Churchill Livingstone威 0883-5403/01/1601-0018$10.00/0 doi:10.1054/arth.2001.16486
107
108 The Journal of Arthroplasty Vol. 16 No. 1 January 2001 proximal tibial bone loss 8 years after TKA. This finding suggests that weakening of the proximal tibial bone also may occur after TKA and contribute to aseptic loosening of the tibial component. The purpose of this study was to determine the effect of stem design on bone density in the proximal tibia using dual-energy x-ray absorptiometry (DEXA) scan analysis in long-term well-functioning TKAs in elderly women. It is unknown whether tibial baseplates of differing geometries with and without tibial stems can modulate the effect seen in the earlier study.
Materials and Methods
Table 1. Summary of Results
Age (y) Height (inches) Weight (lb) Follow-up (mo) Knee Society score Clinical Functional Range of motion (°) Flexion preoperative Flexion postoperative Tibiofemoral alignment (°) Preoperative Postoperative
Miller-Galante
Press-Fit Condylar
71.1 (65–86) 63.8 (62–66) 163 (143–180) 95 (81–98)
73.6 (67–78) 62.5 (61–64) 160 (138–178) 94 (84–101)
93 (88–97) 73 (55–90)
88 (67–98) 67 (45–90)
110 (95–120) 105 (90–115)
110 (100–130) 110 (95–115)
180 (176–184) 174 (173–175)
174 (176–180) 174 (173–175)
NOTE. Values are averages with ranges in parentheses.
The study group comprised 12 asymptomatic healthy white women who underwent TKA for unilateral osteoarthrosis between January 1987 and April 1989 and had a minimal follow-up of 7 years. Patients with contralateral knee pathology or surgery; patients with medical conditions known to alter bone density, such as rheumatoid arthritis; and patients on long-term steroid therapy were excluded. Two implant types with distinct stem designs were used: the Miller-Galante (MG) TKA, (Zimmer, Warsaw IN), with a tibial plate with 4 short (0.5cm) cemented fixation pegs (6 patients), and the Press-Fit Condylar (PFC) TKA (Johnson & Johnson, Rayham, MA), with a 4-cm cemented central stem (6 patients). All perioperative and operative techniques were similar and contemporaneous. All components were cemented in place, including the stems. Postoperative Knee Society knee function and radiograph scores were obtained for all patients. All patients were asymptomatic, and standing radiographs on long plates showed well-positioned prostheses without evidence of loosening or wear. A bilateral DEXA scan of the knees was performed. Bone density was measured at 4 standardized locations (Fig. 1) as described by Levitz et
Fig. 1. Bone density was measured in 4 different areas in the normal control and beneath each prosthesis.
al [8]. For each patient, the ratio of bone density for the operated knee to the corresponding location on the unoperated knee was determined. Statistical analysis was completed with the SAS, including 1-way analysis of variance summary of fit, t-test, and analysis of variance (P ⬍ .05).
Results The average height, weight, and duration of follow-up were similar in both groups (Table 1). The interval from surgery to DEXA scan analysis averaged 95 months in the MG group and 94 months in the PFC group. Knee Society clinical and function scores were 93 and 73 in the MG group and 88 and 67 in the PFC group. These differences were not statistically significant. The preoperative tibiofemoral alignment was similar in the 2 groups (Table 1). Average postoperative alignment was 174° because perfect alignment was required for entry into the study. Two patients used a cane in each group. The bone densities under the medial and lateral plateaus in the stemmed PFC group were significantly reduced as compared with the unstemmed MG group (P ⬍ .001). Distally the diaphyseal densities were not significantly different in each group. Bone density ratios, comparing the tibial bone beneath the medial and lateral plateaus with the nonoperated contralateral tibia were significantly different for each group (P ⬍ .0016). There were no significant differences in the ratios of bone density in the 2 diaphyseal measurements (Table 2).
Bone Density After Cemented TKA • Lonner et al.
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Table 2. Dual-Energy X-Ray Absorptiometry Scan Results Medial Plateau Bone density (g/cm2) MG 0.794 ⫾ 0.121 PFC 0.225 ⫾ 0.097 Contra 0.612 ⫾ 0.072 P value ⬍.001 Ratio to contralateral tibia MG 1.03 ⫾ 0.10 PFC 0.30 ⫾ 0.10 P value .004
Lateral Plateau
Proximal Diaphysis
Distal Diaphysis
0.809 ⫾ 0.162 0.346 ⫾ 0.113 0.670 ⫾ 0.080 ⬍.001
0.793 ⫾ 0.157 0.542 ⫾ 0.139 0.733 ⫾ 0.094 NS
1.06 ⫾ 0.206 0.819 ⫾ 0.093 0.988 ⫾ 0.111 NS
1.27 ⫾ 0.14 0.44 ⫾ 0.14 .0023
0.93 ⫾ 0.13 0.80 ⫾ 0.13 NS
1.0 ⫾ 0.05 0.88 ⫾ 0.05 NS
MG, Miller-Galante; PFC, Press-Fit Condylar; Contra, contralateral tibia.
Discussion Stress shielding and bone resorption after TKA and THA have become a clinical problem and may contribute to prosthesis failure. Bone loss resulting from prosthetic implantation may compromise the stability of the implant itself and make revision surgery more difficult. The results of this study suggest that there is a greater degree of bone loss associated with the use of a tibial component fixed by a cemented stem as compared with an unstemmed design. This finding may have significant implications for the longevity of present TKA designs and for the design of future implants. DEXA scanning has become a useful tool in the determination of bone densities in many locations. With newer computer software packages, the scans are more accurate in the regions around metallic implants. In the proximal femur, accuracy of the DEXA scan is reported to be within 1% for cadaveric specimens and to range from 2% to 6.8% for in vivo measurements [15,16]. Most of the variability in clinical studies is secondary to minor variations in rotation between measurements. Bone density measurements have been shown to correlate closely with the mechanical properties of bone [17– 19], suggesting that bone loss identified by DEXA scan can be a threat to implant stability. At present few data are available on the DEXA measurements around TKAs in vivo, although the data appear similar to the increasing experience in THAs. Robertson et al [28] studied bone loss in a cadaveric distal femur after TKA. Sections of bone measuring 3 mm were removed sequentially, and standard radiographs and DEXA scans were obtained. DEXA scanning was shown to be significantly more accurate than visual interpretation or computer analysis of the radiographs, with a coefficient of variation of 1.2%. The DEXA bone measurements were highly correlated with the calcium
ash content of the bone, with a correlation coefficient of 1. Many TKA designs have used a tibial stem to augment fixation. Initial experience with cemented, all-polyethylene stemmed components showed that although the stems often remained firmly fixed in place, polyethylene deformation caused by eccentric loading led to mechanical failure of the bone in the medial or lateral plateau [21]. Analysis of these tibial components [3,22,23] revealed that there was little axial load bearing through the polyethylene stems, with all stresses being transferred to the proximal bone. As a result of these observations, to protect the proximal bone from failure, metal backing was added to the tibial component. Finite element analysis [3,6] reveals that the addition of metal reduces the overall stresses on the cancellous surface and results in a more even distribution of stresses, especially in eccentric varus or valgus loading conditions. Comparing various designs for fixation of the plate to the bone, Walker et al [24] found that a centrally placed metal stem was found to enhance further resistance to lift-off in eccentric loading as well as improve resistance to shear stresses and was thought to represent the most secure fixation. The axial load bearing by the stem was found to be 28%. Bartel et al [3] concluded that the stem would reduce the overall load carried by the proximal cancellous bone and that the metal tray would improve the distribution of the remaining stresses. Brooks et al [25] and Bourne and Finlay [21] have used arrays of strain gauges to quantitate the stress changes associated with stemmed tibial components, concluding that there is a marked reduction in stresses measured in all locations proximal to the stem. The latter authors speculated that this stress shielding may jeopardize long-term fixation of the tibial component as a result of bone resorption. Whiteside and Pafford [26] evaluated the
110 The Journal of Arthroplasty Vol. 16 No. 1 January 2001 load-bearing capacity of 2 stemmed implants, 1 cemented and 1 press-fit, and found proximal stress shielding of an equal magnitude with each design. Long-term changes in bone density of the distal femur associated with TKA have been documented. Petersen et al [13] found a 36% decrease in bone density behind the anterior femoral flange at 2 years, with a 22% increase in density proximal to the fixation pegs, findings that they attributed to altered mechanical loading conditions. Reports of short-term tibial bone density changes [12,14] revealed little or no change 3 years after TKA [8]. This study compared 2 distinct designs from different manufacturers. The presence or absence of a central tibial stem may not be the sole influence on proximal tibial bone density. Other factors, such as tray thickness and metallurgy, may play a role. The 2 implants tested in this series were both titanium, but platform thickness differed. The MG tray is 3.6 mm thick, and the PFC tray is 2.7 mm thick. Based on this information, one might surmise that the MG tray would be stiffer and stress shield more, if all other variables were controlled perfectly, but we are not aware of tested differences in the stiffness of the baseplates. In addition to these factors and stem geometry, the cement mantle geometry may affect the stress distribution to the underlying bone. This factor was not addressed in this study. The apparent loss of bone density that may result from osteolysis and reaction to particulate debris should be considered. In this series, all patients were asymptomatic, without effusion or evidence of polyethylene wear on standing radiographs. This study supports the contention that use of a cemented stem reduces proximal stresses and may result in proximal bone resorption. This proximal bone loss is not seen in the unstemmed design tested. Although the use of a central stem may enhance component stability, with resistence to lift-off and shear, it comes at a price. The proximal resorption may contribute to the persistence of tibial component loosening as a primary threat to TKA survivorship. This bone loss may complicate revision surgery and can compromise the quality and outcome of a TKA revision. It is unproven that in a cemented tibial component, the stability to lift-off and toggle provided by a central stem is any better than that provided by small pegs [29,30]. In a study by Lonner et al [29], comparing the stability of 4 cemented tibial components with different keel designs, no significant difference was observed in component lift-off or subsidence with application of physiologic cyclic eccentric loads. Two of the implants tested in that earlier study were studied clinically in the present study. In another study,
Stern et al [30] analyzed the micromotion and tilt observed in a single implant design with stem extensions of varying lengths. The authors concluded that in cemented tibial components, in contrast to cementless implants, stem extensions, in the presence of reasonable metaphyseal bone stock, do not enhance initial stability and are unnecessary for most routine primary cemented TKAs [30]. These results are not analogous to the results observed in cementless implants. Implant stability, whether cemented or cementless, is crucial to prevent lift-off, subsidence, and loosening. In the future, consideration should be given to shorter tibial stems or alternate designs that minimize stem fixation for primary cemented TKA.
References 1. Colizza WA, Insall JN, Scuderi GR: The posterior stabilized total knee prosthesis: assessment of polyethylene damage and osteolysis after ten year minimum follow up. J Bone Joint Surg Am 74:1713, 1995 2. Stern SH, Insall JN: Posterior stabilized prosthesis: results after follow-up of nine to twelve years. J Bone Joint Surg Am 74:980, 1992 3. Bartel DH, Burstein AH, Santavicca EA, et al: Performance of the tibial component in total knee replacement. J Bone Joint Surg Am 64:1026, 1982 4. Malkani AL, Rand JA, Bryan RS, et al: Total knee arthroplasty with the kinematic condylar prosthesis: a ten-year follow-up study. J Bone Joint Surg Am 77:423, 1995 5. Moran CG, Pinder IM, Lees TA, et al: Survivorship analysis of the uncemented porous-coated anatomic knee replacement. J Bone Joint Surg Am 73:848, 1991 6. Cohen B, Rushton N: Bone remodelling in the proximal femur after Charnley total hip arthroplasty. J Bone Joint Surg Br 77:815, 1995 7. Hughes SS, Furia JP, Smith P, et al: Atrophy of the proximal part of the femur after total hip arthroplasty without cement: a quantitative comparison of cobaltchromium and titanium femoral stems with use of dual x-ray absorptiometry. J Bone Joint Surg Am 77:231, 1995 8. Levitz CL, Lotke PA, Karp JS: Long-term changes in bone mineral density following total knee replacement. Clin Orthop 321:68, 1995 9. McGovern TF, Engh CA, Zettl-Schaffer K, et al: Cortical bone density of the proximal femur following cementless total hip arthroplasty. Clin Orthop 306: 145, 1994 10. Niinimaki T, Jalovaara P: Bone loss from the proximal femur after arthroplasty with an isoelastic femoral stem: BMD measurements in 25 patients after 9 years. Acta Orthop Scand 66:347, 1995 11. Pritchett JW: Femoral bone loss following hip re-
Bone Density After Cemented TKA • Lonner et al.
12.
13.
14.
15.
16.
17.
18.
19.
20.
placement: a comparative study. Clin Orthop 314: 156, 1995 Bohr HH, Lund B: Bone mineral density of the proximal tibia following uncemented arthroplasty. J Arthroplasty 2:309, 1987 Petersen MM, Olsen C, Lauritzen JB, et al: Changes in bone mineral density of the distal femur following uncemented total knee arthroplasty. J Arthroplasty 10:7, 1995 Seitz P, Ruegsegger P, Gschwend N, et al: Changes in local bone density after total knee replacement. J Bone Joint Surg Br 69:407, 1987 Cohen B, Rushton N: Accuracy of DEXA measurement of bone mineral density after total hip arthroplasty. J Bone Joint Surg Br 77:479, 1995 Trevisan C, Bigoni M, Cherubini R, et al: Dual x-ray absorptiometry for the evaluation of bone density from the proximal femur after total hip arthroplasty: analysis protocols and reproducibility. Calcif Tissue Int 53:158, 1993 Houde J, Marchetti M, Duquette J, et al: Correlation of bone mineral density and femoral neck hardness in bovine and human samples. Calif Tissue Int 57: 201, 1995 Mazess RB, Barden HS: Measurement of bone by dual photon absorptiometry and dual energy x-ray absorptiometry. Ann Chir Gynaecol 77:197, 1988 Nelson D, Feingold M, Mascha E, et al: Comparison of single photon and dual energy x-ray absorptiometry. Bone Miner 18:77, 1992 Ritter MA, Herbst SA, Keating EM, et al: Long-term survival analysis of a posterior cruciate-retaining total condylar total knee arthroplasty. Clin Orthop 309: 136, 1994
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21. Bourne RB, Finlay JB: The influence of tibial component intramedullary stems and implant-cortex contact on the strain distribution of the proximal tibia following total knee arthroplasty. Clin Orthop 208:95, 1986 22. Lewis JL, Askew MJ, Jaycox DP: A comparative evaluation of tibial component designs of total knee prostheses. J Bone Joint Surg Am 64:129, 1982 23. Reilly D, Walker PS, Ben-Dov M, et al: Effects of tibial components on load transfer in the upper tibia. Clin Orthop 165:273, 1982 24. Walker PS, Hsu H, Zimmerman RA: A comparative study of uncemented tibial components. J Arthroplasty 5:245, 1990 25. Brooks PJ, Walker PS, Scott RD: Tibial component fixation in deficient tibial bone stock. Clin Orthop 184:302, 1984 26. Whiteside LA, Pafford J: Load transfer characteristics of a noncemented total knee arthroplasty. Clin Orthop 239:168, 1989 27. Ranawat CS, Flynn WF Jr, Saddler S, et al: Long-term results of the total condylar knee arthroplasty: a 15 year survivorship study. Clin Orthop 286:94, 1993 28. Robertson DD, Mintzer CM, Weissman BN, et al: Distal loss of femoral bone following total knee arthroplasty. J Bone Joint Surg Am 76:66, 1994 29. Lonner JH, Pearlman C, Kummer FJ, et al: The effect of keel design on tibial component stability in total knee arthroplasty. Proceedings of the Scientific Meeting of the Knee Society, San Francisco, February 1997 30. Stern SH, Wills RD, Gilbert JL: The effect of tibial stem design on component micromotion in knee arthroplasty. Clin Orthop 345:44, 1997