Changes in bone mineral density of the distal femur following uncemented total knee arthroplasty

The Journal of Arthroplasty Vol. 10 No. 1 1995

Changes in Bone Mineral Density of the Distal Femur Following U n c e m e n t e d Total Knee Arthroplasty Michael

Mork

Petersen, MD, Claus Olsen, MD, Jes Bruun

Lauritzen,

MD, and

Bjarne Lund, MD, PhD

Abstract: The aim of the study was to quantitate changes in bone mineral density (BMD) in the distal femur following uncemented porous-coated total knee arthroplasty. Eight patients with total knee arthroplasties (PCA Primary, Howmedica, Rutherford, NJ) (female:male ratio, 6:2; mean age, 70 years; range, 51-77 years) were scanned by dual-photon absorptiometry within 3 months after surgery and at 2 years. An average decrease of 36% (P = .01) was found in BMD behind the anterior flange of the femoral prosthesis. Proximal to the fixation pegs, BMD increased by 22% (P = . 12), but behind the posterior flange of the femoral component, BMD remained unchanged (P = .53). Stress shielding anteriorly in the distal femur occurred in all patients examined 2 years after surgery, and the increase in BMD proximal to the fixation pegs was probably a result of increased and altered mechanical loading. Key words: bone mineral density, distal femur, uncemented total knee arthroplasty, stress shielding, osteopenia.

Implantation of total knee arthroplasties (TKAs) m a y induce altered mechanical loading of the distal femoral bone. During revision surgery, bone loss behind the anterior flange of the prosthesis in the femur has been observed. In two retrospective roentgenographic studies, x'x qualitative bone loss anteriorly in the distal femoral condyle has been described (Fig. 1). Functional strain is a w e l l - k n o w n determinant of bone remodeling. 3'4 Bone loss has been s h o w n to occur as a response to immobilization 5'6 and w h e n limbs were immobilized because of fractures. 7"8 Localized bone loss can be induced by stress shielding, which has been observed in the proximal f e m u r after total hip arthroplasty 9"~° and in the distal femur after TKA. x'2,xx Bone density m e a s u r e m e n t s with dualp h o t o n absorptiometry ~2,~3 and c o m p u t e d tomograFrom the Department of Orthopaedic Surgery U, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark. Reprint requests: Michael Mork Petersen, MD, Department of Orthopaedic Surgery U 216 i, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, DK 2100 Copenhagen O, Denmark.

phy ~4 have previously been used to provide information about b o n e reactions in the proximal tibia under the tibial plateau in TKAs. In a prospective followup study, we measured quantitative local changes in bone mineral density (BMD) above the femoral c o m p o n e n t in patients with uncemented, porouscoated TKAs.

Materials and Methods Bone mineral density was measured using dualp h o t o n absorptiometry with a gadolinium 153 source. The BMD scans were performed with a lateral knee scanner (Gammatec GT-50, F e m u r - l a , Vaerlose, Denmark) in the mediolateral plane in the distal femur just above the femoral component. In the c o m p u t e d scan plot, three areas of interest around the fixation pegs were selected for BMD measurements. The first area was located anteriorly to the fixation pegs (BMD 1), the second (BMD 2) was located proximally to the pegs, and the third (BMD

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The Journal of Arthroplasty Vol. 10 No. 1 February 1995

Fig. 1. Lateral roentgenograms of the distal femur immediately after uncemented TKA (left) and 4 years after TKA (right). Note the development of stress shielding in the anterior distal femur.

w e f o u n d a coefficient of variation of 0.5% for BMD m e a s u r e m e n t s . The b o n e s c a n n e r has a built-in softw a r e facility that sets a l o w e r limit for securing reproducible c o u n t i n g statistics, a n d scanning with an old g a d o l i n i u m source c o n s e q u e n t l y increases the total scanning time, but does not affect the c o u n t i n g statis-

3) was located posteriorly to the pegs (Fig. 2). In three patients s c a n n e d twice o n the same day, the precision of the BMD m e a s u r e m e n t s in the distal f e m u r expressed as the coefficient of variation was 1.4% for BMD 1, 1.6% for BMD 2, a n d 2.5% for BMD 3. For repeated scans o n a calcium p h a n t o m ,

BMD 1 (anterior) 1,2

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Fig. 2. On the right, the scan plots of the distal femur over the prosthesis are shown. The areas of interest selected for bone mineral density (BMD) measurements are anterior to the fixation pegs (BMD 1), proximal to the fixation pegs (BMD 2), and posterior to the fixation pegs (BMD 3). On the left, the individual changes in BMD in eight patients with a 2-year follow-up period are shown.

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BMD 2 (proximal) 1,4 BMD (9/cm2) 1,2

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Femoral BMD Following Uncemented TKA

•

Petersen et al.

Table l. Changes in Distal Femoral BMD (g/cmz) After Uncemented TKA in the Follow-up and Cross-sectional Groups Postoperative Time Interval 0-3 Months n Mean age (years) Female/male BMD 1 (anterior) BMD 2 (proximal) BMD 3 (posterior)

5 Years (range, 3-8)

2 Years 8

70 (range, 51-77) 6/2 0.67 (range, 0.37-0.97) 0.43 (range, 0.17-0.63) 0.63 (range, 0.38-0.97) 0.77 (range, 0.31-I.22) 0.81 (range, 0.61-1.21) 0.87 (range, 0.56-1.08)

I1 49 (range, 39-67) 2/9 0.47 (range, 0.09-0.76) 0.78 (range, 0.46-1.20) 0.99 (range, 0.49-1.39)

BMD, bone mineral density.

tics. The radiation dose to the bone marrow at examination was below 0.I mSv. Two groups of patients were included in the study. The first group consisted of eight patients with uncemented, porous-coated total condylar knee arthroplasties (PCA Primary, Howmedica, Rutherford, N J). All of the patients had primary osteoarthrosis. The patients had BMD measurements performed within the first 3 months after the operation and again at 2 years. The second group included ll patients with uncemented PCA Primary prostheses. Five patients had joint destruction secondary to hemophilia and six had primary osteoarthrosis. The patients in the second group had BMD measurements performed only once, but as many as 5 years (range, 3 - 8 years) following the operation. The m e a n age and female:male ratio for the patients in both groups are shown in Table 1. The total fixation surface, including the fixation pegs of the femoral c o m p o n e n t (PCA Primary prosthesis), is porous coated. The u n c e m e n t e d TKA operations were performed as described by Hungerford et al.~ 5 Weight bearing was started 3 weeks after surgery with gradual increases until full weight bearing 3 weeks later.

Statistics Nonparametric Wilcoxon's test for paired data was used, and two-tailed P values less than .05 were considered significant. Results are given in means and total ranges.

Results Among the patients with TKAs w h o had a prospective follow-up evaluation, we found a significant fall (P = .0l) in BMD 1 behind the anterior flange of the femoral component of 36% (Table 1 and Fig. 2). Proximally to the femoral pegs, BMD 2 increased by

22% (P = .i2), and behind the pegs, BMD 3 was unchanged (P = .53) (Table 1 and Fig. 2). In the cross-sectional group in which the patients were measured 5 years (range, 3 - 8 years) after TKA, we did not see any further decrease in BMD. However, the distribution of BMD in the distal femur with a low density anteriorly and a higher BMD proximally and posteriorly to the fixation pegs showed the same pattern as seen in the follow-up group after 2 years. Within the cross-sectional group, the distribution of BMD in the distal femur showed no difference between patients with hemophilia and primary osteoarthrosis.

Discussion We found a significant decrease in BMD, averaging 36%, anteriorly in the distal femur in patients with porous-coated, uncemented TKAs followed for 2 years. The long-term change in BMD measured in patients w h o had their TKAs implanted 3 - 8 years ago indicated no further decrease. This could be explained by differences in age and sex between the two groups. The group with single measurements had a predominance of m e n and a m e a n age that was approximately 20 years younger than the group with 2 years of follow-up evaluations. A new state of equilibrium that adjusted to the altered strain in bone is another explanation. In the region over the femoral pegs, we found an increase in BMD of 22% after 2 years. This increase in BMD may be related to bone remodeling as a response to increased strain. Mintzer et al. 2 found roentgenographically stress shielding in the anterior distal femur in 68% of patients with TKAs. The bone loss was found to be independent of fixation mode (cemented/uncemented) and implant design. The bone loss occurred within the first postoperative year and did not progress further. Cameron et al. ~ also found stress shielding roentgenographically in the distal anterior femur in

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The Journal of Arthroplasty Vol. 10 No. 1 February 1995

almost all cases in a series of 185 cemented TKAs. The stress shielding began at 3 months and stabilized at 2 years after the operation. In contrast to this study, the studies of Mintzer et al. 2 and Cameron et al. 1 were retrospective and based on qualitative observations of roentgenographically detectable bone loss. A change in bone density of at least 2 0 - 3 0 % in the distal anterior femur is required to be detectable roentgenographically. ~6 In a study by Whiteside and Pafford,17 load-transfer characteristics of femoral and tibial components of a total knee prosthesis designed to achieve distal femoral and proximal tibial compressive load bearing were evaluated. Roentgenograms of 110 patients with an uncemented TKA with a follow-up period of 1 2 - 2 4 months were examined; a hypertrophic pattern in the trabecular bone of the distal femur was seen in 93% of the patients and only a few patients showed bone loss in the distal anterior femur. Finite element models of the distal femur with a femoral component ls'19 have s h o w n that strain in the anterior distal femur m a y be altered from a highstress region to a relatively low-stress region after TKA, thus leading to bone loss. The highest degree of stress shielding should be expected if the anterior and posterior flange surfaces are bonded to the bone. Some load can be transferred to the distal anterior femur as compressive strain if the anterior and posterior flange surfaces are not bonded to the bone. This might prevent bone loss in the anterior distal femur following TKA as s h o w n in the clinical roentgenographic study of Whiteside and Pafford.17 By using BMD measurements with dual-photon absorptiometry, it is possible to measure minor changes in bone density with high precision, as well as evaluate the time sequence of bone loss and compare different designs and modes of fixation in TKA. Bone density is related to the breaking strength of bone, 2°,21 and bone loss in the anterior distal femur has been mentioned as a risk factor for supracondylar fractures of the femur following TKA. 22"23 Periprosthetic fractures in TKA are not c o m m o n occurrences, but as early complication and failure rates have diminished, supracondylar fractures of the femur have received increasing attention because they continue to present a treatment dilemma. 24"25 The significant, uneven stress distribution reflected by changes in BMD in the distal femur after TKA m a y be relevant w h e n designing new modified femoral components. As a consequence of the stress shielding observed anteriorly in the distal femur in both this study and previous roentgenographic studies, 1,2 we have modified the anterior and posterior flanges of our femoral components. The flanges are currently being tested in a randomized study to determine the possibility

of secure fixation of the femoral component with a more even stress distribution in the distal femur, thus avoiding the development of stress shielding.

References 1. Cameron HU, Cameron G: Stress relief osteoporosis of the anterior femoral condyles in total knee replacement. Orthop Rev 16:449, 1987 2. Mintzer CM, Robertson DD, Rackemann Set al: Bone loss in the distal anterior femur after total knee arthroplasty. Clin Orthop 260:135, 1990 3. Lanyon LE: Functional strain as a determinant for bone remodeling. Calcif Tissue Int 36:56, 1984 4. Margulies JY, Simkin A, Leichter Iet al: Effect of intense physical activity on the bone mineral content in the lower limbs of young adults. J Bone Joint Surg 68A: 1090, 1986 5. Donaldson CL, Hulley SB, Vogel JM et al: Effect of prolonged bed rest on bone mineral. Metabolism 19: 1071, 1970 6. Krolner B, Toft B: Vertebral bone loss: an unheeded side effect of therapeutic bed rest. Clin Sci 64:537, 1983 7. Andersson SM, Nilsson BE: Changes in bone mineral content following tibia shaft fractures. Clin Orthop 144:226, 1979 8. Petersen MM, Olsen C, Lauritzen JB, Lund B: Changes in bone mineral content in the proximal tibia following ankle fracture. Eur J Exp Musculoskel Res 1:77, 1992 9. Brown IW, Ring PA: Osteolytic changes in the upper femoral shaft following porous coated hip replacement. J Bone Joint Surg 67B:218, 1985 10. Kiratli BJ, Heiner JP, McKinley Net al: Bone mineral density of the proximal femur after uncemented total hip arthroplasty. Trans Orthop Res Soc 16:545, 1991 11. Bobyn JD, Cameron HU, Abdulla D et al: Biologic fixation and bone modeling with an unconstrained canine total knee prosthesis. Clin Orthop 166:301, 1982 12. Bohr HH, Lund B: Bone mineral density of the proximal tibial following uncemented arthroplasty. J Arthroplasty 2:309, 1987 13. Bohr HH, Schaadt O: Mineral content of upper tibia assessed by dual photon densitometry. Acta Orthop Scand 58:557, 1987 14. Hvid I, Bentzen SM, Jorgensen J: Remodeling of the tibial plateau after knee replacement. Acta Orthop Scand 59:567, 1988 15. Hungerford DS, Kenna RV, Krackow KA: The porous coated anatomic total knee. Orthop Clin North Am 13:103, 1982 16. Finsen V, Anada S: Accuracy of visually estimated bone mineralization in routine radiographs of the lower extremity. Skeletal Radiol 17:270, 1988 17. Whiteside LA, Pafford J: Load transfer characteristics of a noncemented total knee arthroplasty. Clin Orthop 239:168, 1989

Femoral BMD Following Uncemented TKA

18. Tissakht M, Ahmed AM, Chan KC: Stress shielding in the distal femur following TKR: effect of bone/implant interface condition. Trans Orthop Res Soc 18:426, 1993 19. Walker PS, Granholm J, Lowrey R: The fixation of femoral components of condylar knee prosthesis. Eng Med 11:135, 1982 20. Smith CB, Smith DA: Relations between age, mineral density and mechanical properties of human femoral compacta. Acta Orthop Scand 47:496, 1976 2I. Stromsoe K, Alho A, Kok WL, Hoiseth A: The relation between mechanical properties and bone mineral of condylar cancellous bone of the femur. Eur J Exp Musculoskel Res 3:17, 1994

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22. Hirsh DM, Bhalla S, Roffman M: Supracondylar fracture of the femur following total knee replacement. J Bone Joint Surg 63A:I62, 1981 23. Kraay MJ, Goldberg VM, Figgie MP, Figgie HE III: Distal femoral replacement with allograft/prosthetic reconstrutcion for treatment of supracondylar fractures in patients with total knee arthroplasty. J Arthroplasty 7:7, 1992 24. Cordeiro EN, Costa RC, Carazzato JG, Silva JDS: Periprosthetic fractures in patients with total knee arthroplasties. Clin Orthop 252:182, I990 25. Figgie MP, Goldberg VM, Figgie HE III, Sobel M: The results of treatment of supracondylar fracture above total knee arthroplasty. J Arthroplasty 5:267, 1990