- Email: [email protected]
A new approach of designing the tibial baseplate of total knee prostheses
Clinical Biomechanics
14 (1999) 112-l 17
A new approach of designing the tibia1 baseplate of total knee prostheses Cheng-Kung Cheng”, Chen-Yu Lung”, Ye-Ming Leeb, Chun-Hsiung Huangb* “Orthopaedic Biomechanics Laboratory, Institute of Biomedical Engineering, National Yang Ming University, Republic of China hDepanment of Orthopaedic Sutgev, Mackay Memorial Hospital, 92, Sec. 3, Chung-San N Road, Taipei, Taiwan, Republic of China Received 15 December 1997; accepted 17 June 1998
Abstract Objective. To improve the design of the, knee prosthesis, a new technique to design the tibia1 baseplate of total knee prostheses was developed. Methods. One senior surgeon operated on 79 osteoarthrosis patients by using PCA total knee prostheses for total knee replacement. Four dimensions were measured intraoperatively and compared, including the anterioposterior length and the mediolateral width of the resected tibia plateau and implant. The data of anterioposterior length and the mediolateral width of resected surfaces of the tibia1 plateau of the 79 patients were plotted in a coordinate system. This coordinate system was formed with the mediolateral width as the x-axis and anterioposterior length as the y-axis. A circle, 5 mm in diameter, was used as the maximum coverage criterion. Five contemporary products and a new design product were used to screen the data to see how many patients would fall within the criteria. From the results of the screened data, we calculated the coverage percentage of the patients. Results. The ratio of the anterioposterior length to the mediolateral width of the resected surfaces of the tibia1 plateau was greater than that of the ratio of the PCA prostheses (P
Relevance In total knee replacement, the shape of the knee prostheses is important because many complications may result from a mismatch between the implant and bones. Most knees that required total knee replacement are diseased knees, with shapes quite different from those of normal knees. Since good coverage of bone was based on the resected tibia1 plateau and related implants, we suggest that the dimensions of the resected surfaces of the OA knees should be considered as an important factor in the design of the tibia1 baseplate. A new method and technique were thus developed to find out the ratios of the anterioposterior length to the mediolateral width with minimum sizes and these sizes that can cover at least 90% of the patients. 0 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tibia1 baseplate; Knee dimensions; Tibia1 plateau; Osteoarthritic
knee; Resected surface
1. Introduction
smaller than the resected surface of the bone, it may
In total knee replacement, the shape of the knee prosthesis is very important because many complications may arise from a mismatch between the prostheses and the bones [l-3]. If the prosthesis is *Corresponding
author. E-mail: [email protected]
subside. While if it is larger than the resected surface of the bone, it may impinge on the surrounding soft tissues. To improve the design of knee prostheses, knee shapes have been widely studied. Many researchers have studied normal knee morphology from cadavers and X-ray films [4-lo]. However, most knees that
026%0033/99/$ - see front matter 0 1999 Elsevier Science Ltd. All rights reserved. PII: SO268-0033(98)00054-O
C-K. Cheng et al./Clinical
Biomechanics 14 (1999) 112-117
require total knee replacement are diseased with shapes quite different from those of normal knees. Since better coverage between implant and cutting bone is desired, data regarding resected surfaces of the tibia plateau should be used to improve the design of the tibia1 baseplate of total knee prostheses. In addition, the geometry of the resected surface of the tibia plateau varies considerably on the level of which the cut is made. Therefore, the objective of this study was to measure the resected surfaces of the tibia1 plateau of diseas.edknees and make a good estimation of sizing the tibia1 baseplate for total knee replacement. 2. Methods From October 1994 to August 1995, 79 patients underwent total knee replacement surgery with prostheses by PCA modular total knee system (Howmedica, Ru.therford, NJ, USA). The basic information regarding these patients is listed in Table 1. The mean age was 66.3 years. The mean weight was 64.4 kg. Out of the 79 patients, 74 were female, 5 were male. All patients were Chinese and osteoarthritic. The prostheses were implanted by an experienced senior surgeon. The instruments were provided by the Howmedica company. All knees were measured during surgery after the tibia1 plateau had been cut. Four dimensions were measured, including the anterioposterior length (AP) and the mediolateral width (ML) of both of the resected surface of the tibia1 plateau and the tibia1 baseplate of the ;prosthesis. The AP and the ML were measured to the longest dimension with a slide caliper made by the Mitsutoyo company (Tokyo, Japan). Anterior and proximal views of the resected surface of the tibia1 plateau. are shown in Fig. 1. All measurements were carried out by a senior surgeon, who has performed more than 1500 casesof total knee replacement. The reliability of these measurements was checked by measuring the dimensions of tibia baseplates on the table. The AP and the ML dimensions of the same sized prostheses implanted in different patients were measured and compared (Table 2). The paired t-test was used to calculate the statistical significance of any difference. The average Table 1 Basic information Sex
Total Male Female
113
Fig. 1. The AP and ML dimensions in the anterior views of the resected surface of osteoarthritic knees.
and superior
values of the ratio of AP to ML of five other companies’ products were also calculated from the data listed in their catalog. They were MG II (Zimmer, Warsaw, IN, USA), Ortholoc (Wright, Arlington, TN, USA), Omnifit-Series 7000 (Osteonics, Allendale, NJ, USA), Performance (Kirschner, Timonium, ML, USA) and Duracon (Howmedica, Rutherford, NJ, USA). We compared the dimensions of the resected surface of the tibia1 plateau with the dimensions of these implants. The ML and the AP data of the resected surface of the tibia1 plateau of the 79 patients were plotted in a coordinate system. This coordinate system was created with ML as the x-axis and AP as the y-axis (Fig. 2). A screening technique was used to determine which specific prosthetic tibia1 baseplate could cover the Table 2 Errors of measurement (unit: mm)
Dimensions listed on the catalog of PCA Average of the 16 measurements Standard errors
ML
AP
67 67.1 0.19
42 41.0 0.72
AP(mm) soY 55 I I 50 .
+
+
*
+;Xp+ +++* + * 44 +++ ++e+ ** +++++++ +
45 . 40.
on the subjects
Subject number
Age (years)
Height (cm)
Weight (kg)
79 5 74
66.3 (5.9) 72.6 (7.0) 65.8 (5.5)
153.5 (5.8) 157.0 (8.0) 152.2 (5.3)
64.4 (8.2) 60.5 (8.7) 64.8 (8.2)
60
70
80
80
Mumm) Fig. 2. Dimensions of AP and ML of resected surfaces of osteoarthritic knees with mediolateral width as the x-axis and anterioposterior length as the y-axis.
114
C-K. Cheng et d/Clinical Biomechanics 14 (1999) 112-117
maximum number of resected surfaces. Erkman and Walker [7] have suggested using 5 mm as the size gap to differentiate sizes. They thought if the AP and the ML data of any patient were greater or less than the implant by 5 mm, the bone could not have better coverage by the implant. This criterion can get better coverage of the data made by normal knees [7]. This technique uses circles with 5 mm diameters to screen the data. The center of these circles were determined by the dimensions of AP and ML of the tibia1 baseplates of the prostheses of the five companies previously mentioned. The data of the patients that fell into these circles were counted. Then the number and percentage were calculated to see which company could best meet the criterion of coverage for these patients. The criterion for better coverage between resected surface of bone and implant is that both the dimensions of AP and ML of bone should be less than the implant by 5 mm. Using the same technique, we determined the minimum number of sizes of tibia1 baseplate by analysis with one self-designed software. This minimum number of sizes of tibia baseplates can cover 90% of the data. The data of the resected surface of tibia1 plateau in this study were also compared with those of normal cadaver knees published in various literatures [5-7,9]. 3. Results The AP and the ML dimensions of the resected surface of the tibia1 plateau and tibia1 baseplates of PCA prostheses are shown in Fig. 3. The ML of the resected surface of the tibia1 plateau and that of the implant were almost the same (P > 0.1). However, the AP of the tibia1 baseplate was smaller than the 80
70
60
50
40
om!!Jfil
oIuI0,cc
MGU
hracon
Rrhnnancc
Rsxeadnnfnar
Fig. 4. Comparison of AP-to-ML ratios from resected surface of osteoarthritic knees with those tibia1 baseplates of five different prostheses.
resected surface of the tibia1 plateau (PC 0.05). As a result, the ratio of AP to ML of the resected tibia1 plateau surfaces was greater than that of the PCA prostheses (PC 0.05). Even greater discrepancies were found between the resected surface dimensions and those of the other five prostheses (Fig. 4). Using the 5-mm circle screening technique, the coverage percentages of those five products were quite low. The lowest coverage percentage was 20% for Kirschner’s Performance implants and the highest was only 70% for Howmedica’s Duracon implants, no matter how many different sizes of prostheses were used (Fig. 5(a-e)). The new designed tibia1 baseplate with five sizes .of different ratio of AP-to-ML were decided. These different ratios of AP-to-ML were chosen to achieve 90% coverage of the OA patients that met the criterion (Fig. 6). The AP-to-ML ratios of the tibia1 plateau of normal cadaver knees reported in the literature [5-7,9] were also apparently different from the ratio obtained in this study (Fig. 7). To confirm the reliability of our measurements, we used the same technique to measure 16 prostheses of the same size (Table 2). The mean AP was 41.0 (SD, 0.72) mm. It was a very small standard deviation, only 0.72 mm. It showed that our measurements were quite reliable.
30
4. Discussion
20
IO 0 APML (unit:%)
Fig. 3. Comparison of dimensions of resected surface of osteoarthritic knees and of tibia1 baseplates of PCA prostheses.
After total knee replacement, the prosthesis makes contact with the resected surfaces of bone, resulting in some degree of contact stress [ll-141. If there is a good match, the contact area will be larger, and the contact stress will be lower. Conversely, if there is a poor match, the contact area will be smaller and the
C-K. Cheng et al.IClinical
contact stress will be higher, which will result in loosening of the knee prostheses. The unequal coverage of the resected bone by implant not only causes the higher stress but also results in loosening. Worn polyethylene particles may drop onto the raw AP(mm) 60
surfaces of the resected bones, possibly inducing osteolysis during knee movement [15]. In addition, the uneven stress distribution on the tibia plateau may lead to uneven bone density and strength, resulting in further loosening of the prostheses.
AP(mm) 60
( 56 %)
55
55
50
50
45
45 1
40
40 1
L
60
60
70
(4
90
70
80
90
80
90
ML(mm)
AP(mm) 60
( 70 %)
55 El
50
40
o
+# + x+ +* A;“,:p &+=’ m+# ++++
@I
55
45
(48%)
60
ML(mm)
AP(mm) 60
115
Biomechanics 14 (1999) 112-117
50 45
0
40
60
60
70
90
ML(mm)
(cl
60 (4
70 ML(mm)
AP(mm) 60 I
60 (e)
80
70
90
ML(mm)
Fig. 5. Percentages of coverage of the patients of five different prostheses by using S-mm diameter circle screening criterion: (a) Omnifit-Series 7000, (b) Ortholoc, (c) MG II, (d) Duracon, and (e) Performance.
116
C-K. Cheng et aLlClinical Biomechanics 14 (1999) 112-117
Fig. 6. Comparison of AP-to-ML ratios from cadaver studies with resected surfaces of osteoarthritic knees.
Discrepancy between the resected surface of the tibia1 plateau and the tibia1 baseplate of the implant might occur in arthritic knees that have deformed, resulting in different knee dimensions than those obtained from normal cadaver knees. Deformation of the tibia1 plateau and femoral condyle has been observed in both valgus and varus knees. In our subjects, 92% had a varus deformity, in which the greater force in the medial compartment induced deformation and a wider medial tibia1 plateau. Thus, the geometry of the diseased bone at the cutting level is different from normal knees. The modified AP length not only supplies good coverage of the resected surface but also provides a longer roll-back translation length when the knee does the flexion-extension movement. The increased AP length will increase the stability of the knee prostheses when the knee joint is balanced by a serious contraction of the posterior crucial ligment. The increased AP length will also reduce the stress concentration effect and heat effect on the tibia1 insert by normally distriAP(mm) 60
(90%)
i 55
60
70
60
90
ML(mm) Fig. 7. The minimum sizes of the different cover 90% of the OA patients.
AP-to-ML
ratio that can
buting the load in a longer translation length in the sagittal plane. We compared the AP-to-ML ratio of the resected tibia1 plateau surfaces to several reports from literature on cadaver knees (Fig. 7). It included two groups of Chinese knees [5,9] and two groups of Caucasian knees [6,7]. There appeared to be no difference in the AP-toML ratio between Chinese and Caucasian cadaver knees. However, our measurements in diseased knees yielded a higher AP-to-ML ratio than in any other cadaver studies. The AP-to-ML ratio obviously varies in the level at which the cut is made. For total knee arthroplasty, the tibia1 plateau is cut at a thickness of 8 mm. At this level, the slope of the anterior tibia1 edge results in a longer AP dimension, thus generating a greater AP-toML ratio. It remains to be determined whether the differences could all be attributed to the level at which the bone was cut. The differences between Asian and Caucasian bone conformation might explain the differences between the resected surface data and published data. However, this is suspect since results of cadaver studies in both populations were not significantly different [lo]. For total knee arthroplasty, the important dimensions are those of the tibia1 plateau at the level that the bone is cut, because the match situation between the bone surface and prostheses will result in different contact stress which causes osteolysis. Deformity caused by arthritis may require the surgeon to adjust the level of the cut. Therefore, prostheses of various sizes are needed. The design of the prosthetic tibia1 baseplate should be based on data from resected diseased knees, rather than normal knees. How to decide how many different sizes of tibia1 baseplates should be manufactured has been a dilemma in the past. More sizes can lower the gap between each size, and thus every patient can get better coverage between bone and implant. But this will increase the cost and disturb most of the hospitals and orthopaedic companies. And more sizes are not the only method to provide more patients with better bone coverage under certain criterion. In our analysis, the ratio of AP-to-ML of the tibia1 baseplate seems to dominate the choice of size. Even when the company has over ten sizes, like the Howmedica Company, this still does not allow most patients to get good bone coverage under certain criteria. If the data of certain groups can be collected, the minimum number of sizes can be chosen under certain criteria. Another important step is to consider the AP and ML of the patient simultaneously. When the surgeon decides the cutting thickness of the tibia1 plateau, the relationship between AP and ML is determined simultaneously. What this means is that the size ratio should be decided upon the distribution of the patients’ data of
C-K. Cheng et al.lClinical
Biomechanics 14 (1999) 112-117
AP and ML rather than the mean value and standard deviation of AP and ML separately. In this study, about 92% of subjects were female which may have tsontributed to some bias in the results. Further investigation involving a larger number of patients, including more male subjects, needs to be conducted. 5. Conclusion
The measurements of resected bones as performed in this study provides a new approach to evaluating the suitability of total knee prostheses. There are two important points to consider: (1) the measurement of diseased bones is more suitable than that of the normal bones since they are the ones which require total knee replacement; (2) the surface of the resected bone making contact with the prosthesis should be considered more important than the superficial position. Therefore, dimensions of the resected surfaces of the OA knees should be considered as an important factor in the design of the tibia1 baseplate. References [l] Freeman M.A.R., Todd R.C., Bamert P., Day W.H.. ICLH arthroplasty of t:he knee: 1968-1977. J Bone and Joint Surg 1978;60B:339-344. [2] Insall JN. Revision of aseptic failed total knee arthroplasty. In: Insall JN, editor. Surgery of the Knee 2nd, ed. Churchill Livingston, New York, 1994:935-57.
117
[3] Krackow KA. Prostheses selection. The technique of total knee arthroplasty. C.V. Mosby, St Louis, 1991:49-74. [4] Insall JN. Surgical techniques and instrumentation in total knee arthroplasty. In: Insall JN. Surgery of the knee. 2nd ed. Churchill Livingston, New York, 1994:935-57. [5] Liu H.C.. Review of gross anatomy of the Chinese knee. J Formosan Med Assoc 1984;83:317-325. [6] Mensch J.S., Amstuta H.C.. Knee morphology as a guide to knee replacement. Clin Orthop 1975;112:231-241. [7] Erkman MJ, Walker PS. A study of knee geometry applied to the design of condylar prostheses. Biom Eng 1974; January: 9-14. [B] Seedham BB et al. Dimensions of the knee. Ann Rheum Dis 1972: 31-54. [9] Wang S.W., Feng C.H., Lu H.S.. Original articles: a study of the Chinese knee joint geometry for prostheses design. Chinese Medical Journal 1992;105(3):227-233. IO] Ishinishi T, Ogata K, Nishino I, White SE, Whiteside LA. Comparison of Asian and Caucasian normal and osteoarthritic knees. In: 40th Ann Meeting, Ortho Research Sociey, 1994: 663-4. 1l] Little R.B., Wever H.W., Siu D., Cooke T.D.. A three-dimensional finite element analysis of the upper tibia. J Biomech Eng 1986;108(2):111-119. 121 Schreppers G.J., Sauren A.A., Huson A.. A numerical model of the load transmission in the tibia-femoral contact area. Proceeding of the Institution of Mechanical Engineers. Art H-J of Engineering in Medicine 1990;204( 1):53-59. [13] Walker P.S.. Design of Kinemax total knee replacement, bearing surface. Acta Orthop Belgica 1991;57(supple 2): 108-113. [14] Whiteside L.A., Pafford J.. Load transfer characteristics of a non-cemented total knee arthroplasty. Clin Orthop 1989;239: 168-177. [15] Ward W.G., Johnston K.S., Dorey F.J., Eckardt J.J.. Extramedullary porous coating to prevent diaphyseal osteolysis and radiolucent lines around proximal tibia1 replacements. J Bone and Joint Surg 1993;75-A:976-987,
14 (1999) 112-l 17
A new approach of designing the tibia1 baseplate of total knee prostheses Cheng-Kung Cheng”, Chen-Yu Lung”, Ye-Ming Leeb, Chun-Hsiung Huangb* “Orthopaedic Biomechanics Laboratory, Institute of Biomedical Engineering, National Yang Ming University, Republic of China hDepanment of Orthopaedic Sutgev, Mackay Memorial Hospital, 92, Sec. 3, Chung-San N Road, Taipei, Taiwan, Republic of China Received 15 December 1997; accepted 17 June 1998
Abstract Objective. To improve the design of the, knee prosthesis, a new technique to design the tibia1 baseplate of total knee prostheses was developed. Methods. One senior surgeon operated on 79 osteoarthrosis patients by using PCA total knee prostheses for total knee replacement. Four dimensions were measured intraoperatively and compared, including the anterioposterior length and the mediolateral width of the resected tibia plateau and implant. The data of anterioposterior length and the mediolateral width of resected surfaces of the tibia1 plateau of the 79 patients were plotted in a coordinate system. This coordinate system was formed with the mediolateral width as the x-axis and anterioposterior length as the y-axis. A circle, 5 mm in diameter, was used as the maximum coverage criterion. Five contemporary products and a new design product were used to screen the data to see how many patients would fall within the criteria. From the results of the screened data, we calculated the coverage percentage of the patients. Results. The ratio of the anterioposterior length to the mediolateral width of the resected surfaces of the tibia1 plateau was greater than that of the ratio of the PCA prostheses (P
Relevance In total knee replacement, the shape of the knee prostheses is important because many complications may result from a mismatch between the implant and bones. Most knees that required total knee replacement are diseased knees, with shapes quite different from those of normal knees. Since good coverage of bone was based on the resected tibia1 plateau and related implants, we suggest that the dimensions of the resected surfaces of the OA knees should be considered as an important factor in the design of the tibia1 baseplate. A new method and technique were thus developed to find out the ratios of the anterioposterior length to the mediolateral width with minimum sizes and these sizes that can cover at least 90% of the patients. 0 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tibia1 baseplate; Knee dimensions; Tibia1 plateau; Osteoarthritic
knee; Resected surface
1. Introduction
smaller than the resected surface of the bone, it may
In total knee replacement, the shape of the knee prosthesis is very important because many complications may arise from a mismatch between the prostheses and the bones [l-3]. If the prosthesis is *Corresponding
author. E-mail: [email protected]
subside. While if it is larger than the resected surface of the bone, it may impinge on the surrounding soft tissues. To improve the design of knee prostheses, knee shapes have been widely studied. Many researchers have studied normal knee morphology from cadavers and X-ray films [4-lo]. However, most knees that
026%0033/99/$ - see front matter 0 1999 Elsevier Science Ltd. All rights reserved. PII: SO268-0033(98)00054-O
C-K. Cheng et al./Clinical
Biomechanics 14 (1999) 112-117
require total knee replacement are diseased with shapes quite different from those of normal knees. Since better coverage between implant and cutting bone is desired, data regarding resected surfaces of the tibia plateau should be used to improve the design of the tibia1 baseplate of total knee prostheses. In addition, the geometry of the resected surface of the tibia plateau varies considerably on the level of which the cut is made. Therefore, the objective of this study was to measure the resected surfaces of the tibia1 plateau of diseas.edknees and make a good estimation of sizing the tibia1 baseplate for total knee replacement. 2. Methods From October 1994 to August 1995, 79 patients underwent total knee replacement surgery with prostheses by PCA modular total knee system (Howmedica, Ru.therford, NJ, USA). The basic information regarding these patients is listed in Table 1. The mean age was 66.3 years. The mean weight was 64.4 kg. Out of the 79 patients, 74 were female, 5 were male. All patients were Chinese and osteoarthritic. The prostheses were implanted by an experienced senior surgeon. The instruments were provided by the Howmedica company. All knees were measured during surgery after the tibia1 plateau had been cut. Four dimensions were measured, including the anterioposterior length (AP) and the mediolateral width (ML) of both of the resected surface of the tibia1 plateau and the tibia1 baseplate of the ;prosthesis. The AP and the ML were measured to the longest dimension with a slide caliper made by the Mitsutoyo company (Tokyo, Japan). Anterior and proximal views of the resected surface of the tibia1 plateau. are shown in Fig. 1. All measurements were carried out by a senior surgeon, who has performed more than 1500 casesof total knee replacement. The reliability of these measurements was checked by measuring the dimensions of tibia baseplates on the table. The AP and the ML dimensions of the same sized prostheses implanted in different patients were measured and compared (Table 2). The paired t-test was used to calculate the statistical significance of any difference. The average Table 1 Basic information Sex
Total Male Female
113
Fig. 1. The AP and ML dimensions in the anterior views of the resected surface of osteoarthritic knees.
and superior
values of the ratio of AP to ML of five other companies’ products were also calculated from the data listed in their catalog. They were MG II (Zimmer, Warsaw, IN, USA), Ortholoc (Wright, Arlington, TN, USA), Omnifit-Series 7000 (Osteonics, Allendale, NJ, USA), Performance (Kirschner, Timonium, ML, USA) and Duracon (Howmedica, Rutherford, NJ, USA). We compared the dimensions of the resected surface of the tibia1 plateau with the dimensions of these implants. The ML and the AP data of the resected surface of the tibia1 plateau of the 79 patients were plotted in a coordinate system. This coordinate system was created with ML as the x-axis and AP as the y-axis (Fig. 2). A screening technique was used to determine which specific prosthetic tibia1 baseplate could cover the Table 2 Errors of measurement (unit: mm)
Dimensions listed on the catalog of PCA Average of the 16 measurements Standard errors
ML
AP
67 67.1 0.19
42 41.0 0.72
AP(mm) soY 55 I I 50 .
+
+
*
+;Xp+ +++* + * 44 +++ ++e+ ** +++++++ +
45 . 40.
on the subjects
Subject number
Age (years)
Height (cm)
Weight (kg)
79 5 74
66.3 (5.9) 72.6 (7.0) 65.8 (5.5)
153.5 (5.8) 157.0 (8.0) 152.2 (5.3)
64.4 (8.2) 60.5 (8.7) 64.8 (8.2)
60
70
80
80
Mumm) Fig. 2. Dimensions of AP and ML of resected surfaces of osteoarthritic knees with mediolateral width as the x-axis and anterioposterior length as the y-axis.
114
C-K. Cheng et d/Clinical Biomechanics 14 (1999) 112-117
maximum number of resected surfaces. Erkman and Walker [7] have suggested using 5 mm as the size gap to differentiate sizes. They thought if the AP and the ML data of any patient were greater or less than the implant by 5 mm, the bone could not have better coverage by the implant. This criterion can get better coverage of the data made by normal knees [7]. This technique uses circles with 5 mm diameters to screen the data. The center of these circles were determined by the dimensions of AP and ML of the tibia1 baseplates of the prostheses of the five companies previously mentioned. The data of the patients that fell into these circles were counted. Then the number and percentage were calculated to see which company could best meet the criterion of coverage for these patients. The criterion for better coverage between resected surface of bone and implant is that both the dimensions of AP and ML of bone should be less than the implant by 5 mm. Using the same technique, we determined the minimum number of sizes of tibia1 baseplate by analysis with one self-designed software. This minimum number of sizes of tibia baseplates can cover 90% of the data. The data of the resected surface of tibia1 plateau in this study were also compared with those of normal cadaver knees published in various literatures [5-7,9]. 3. Results The AP and the ML dimensions of the resected surface of the tibia1 plateau and tibia1 baseplates of PCA prostheses are shown in Fig. 3. The ML of the resected surface of the tibia1 plateau and that of the implant were almost the same (P > 0.1). However, the AP of the tibia1 baseplate was smaller than the 80
70
60
50
40
om!!Jfil
oIuI0,cc
MGU
hracon
Rrhnnancc
Rsxeadnnfnar
Fig. 4. Comparison of AP-to-ML ratios from resected surface of osteoarthritic knees with those tibia1 baseplates of five different prostheses.
resected surface of the tibia1 plateau (PC 0.05). As a result, the ratio of AP to ML of the resected tibia1 plateau surfaces was greater than that of the PCA prostheses (PC 0.05). Even greater discrepancies were found between the resected surface dimensions and those of the other five prostheses (Fig. 4). Using the 5-mm circle screening technique, the coverage percentages of those five products were quite low. The lowest coverage percentage was 20% for Kirschner’s Performance implants and the highest was only 70% for Howmedica’s Duracon implants, no matter how many different sizes of prostheses were used (Fig. 5(a-e)). The new designed tibia1 baseplate with five sizes .of different ratio of AP-to-ML were decided. These different ratios of AP-to-ML were chosen to achieve 90% coverage of the OA patients that met the criterion (Fig. 6). The AP-to-ML ratios of the tibia1 plateau of normal cadaver knees reported in the literature [5-7,9] were also apparently different from the ratio obtained in this study (Fig. 7). To confirm the reliability of our measurements, we used the same technique to measure 16 prostheses of the same size (Table 2). The mean AP was 41.0 (SD, 0.72) mm. It was a very small standard deviation, only 0.72 mm. It showed that our measurements were quite reliable.
30
4. Discussion
20
IO 0 APML (unit:%)
Fig. 3. Comparison of dimensions of resected surface of osteoarthritic knees and of tibia1 baseplates of PCA prostheses.
After total knee replacement, the prosthesis makes contact with the resected surfaces of bone, resulting in some degree of contact stress [ll-141. If there is a good match, the contact area will be larger, and the contact stress will be lower. Conversely, if there is a poor match, the contact area will be smaller and the
C-K. Cheng et al.IClinical
contact stress will be higher, which will result in loosening of the knee prostheses. The unequal coverage of the resected bone by implant not only causes the higher stress but also results in loosening. Worn polyethylene particles may drop onto the raw AP(mm) 60
surfaces of the resected bones, possibly inducing osteolysis during knee movement [15]. In addition, the uneven stress distribution on the tibia plateau may lead to uneven bone density and strength, resulting in further loosening of the prostheses.
AP(mm) 60
( 56 %)
55
55
50
50
45
45 1
40
40 1
L
60
60
70
(4
90
70
80
90
80
90
ML(mm)
AP(mm) 60
( 70 %)
55 El
50
40
o
+# + x+ +* A;“,:p &+=’ m+# ++++
@I
55
45
(48%)
60
ML(mm)
AP(mm) 60
115
Biomechanics 14 (1999) 112-117
50 45
0
40
60
60
70
90
ML(mm)
(cl
60 (4
70 ML(mm)
AP(mm) 60 I
60 (e)
80
70
90
ML(mm)
Fig. 5. Percentages of coverage of the patients of five different prostheses by using S-mm diameter circle screening criterion: (a) Omnifit-Series 7000, (b) Ortholoc, (c) MG II, (d) Duracon, and (e) Performance.
116
C-K. Cheng et aLlClinical Biomechanics 14 (1999) 112-117
Fig. 6. Comparison of AP-to-ML ratios from cadaver studies with resected surfaces of osteoarthritic knees.
Discrepancy between the resected surface of the tibia1 plateau and the tibia1 baseplate of the implant might occur in arthritic knees that have deformed, resulting in different knee dimensions than those obtained from normal cadaver knees. Deformation of the tibia1 plateau and femoral condyle has been observed in both valgus and varus knees. In our subjects, 92% had a varus deformity, in which the greater force in the medial compartment induced deformation and a wider medial tibia1 plateau. Thus, the geometry of the diseased bone at the cutting level is different from normal knees. The modified AP length not only supplies good coverage of the resected surface but also provides a longer roll-back translation length when the knee does the flexion-extension movement. The increased AP length will increase the stability of the knee prostheses when the knee joint is balanced by a serious contraction of the posterior crucial ligment. The increased AP length will also reduce the stress concentration effect and heat effect on the tibia1 insert by normally distriAP(mm) 60
(90%)
i 55
60
70
60
90
ML(mm) Fig. 7. The minimum sizes of the different cover 90% of the OA patients.
AP-to-ML
ratio that can
buting the load in a longer translation length in the sagittal plane. We compared the AP-to-ML ratio of the resected tibia1 plateau surfaces to several reports from literature on cadaver knees (Fig. 7). It included two groups of Chinese knees [5,9] and two groups of Caucasian knees [6,7]. There appeared to be no difference in the AP-toML ratio between Chinese and Caucasian cadaver knees. However, our measurements in diseased knees yielded a higher AP-to-ML ratio than in any other cadaver studies. The AP-to-ML ratio obviously varies in the level at which the cut is made. For total knee arthroplasty, the tibia1 plateau is cut at a thickness of 8 mm. At this level, the slope of the anterior tibia1 edge results in a longer AP dimension, thus generating a greater AP-toML ratio. It remains to be determined whether the differences could all be attributed to the level at which the bone was cut. The differences between Asian and Caucasian bone conformation might explain the differences between the resected surface data and published data. However, this is suspect since results of cadaver studies in both populations were not significantly different [lo]. For total knee arthroplasty, the important dimensions are those of the tibia1 plateau at the level that the bone is cut, because the match situation between the bone surface and prostheses will result in different contact stress which causes osteolysis. Deformity caused by arthritis may require the surgeon to adjust the level of the cut. Therefore, prostheses of various sizes are needed. The design of the prosthetic tibia1 baseplate should be based on data from resected diseased knees, rather than normal knees. How to decide how many different sizes of tibia1 baseplates should be manufactured has been a dilemma in the past. More sizes can lower the gap between each size, and thus every patient can get better coverage between bone and implant. But this will increase the cost and disturb most of the hospitals and orthopaedic companies. And more sizes are not the only method to provide more patients with better bone coverage under certain criterion. In our analysis, the ratio of AP-to-ML of the tibia1 baseplate seems to dominate the choice of size. Even when the company has over ten sizes, like the Howmedica Company, this still does not allow most patients to get good bone coverage under certain criteria. If the data of certain groups can be collected, the minimum number of sizes can be chosen under certain criteria. Another important step is to consider the AP and ML of the patient simultaneously. When the surgeon decides the cutting thickness of the tibia1 plateau, the relationship between AP and ML is determined simultaneously. What this means is that the size ratio should be decided upon the distribution of the patients’ data of
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AP and ML rather than the mean value and standard deviation of AP and ML separately. In this study, about 92% of subjects were female which may have tsontributed to some bias in the results. Further investigation involving a larger number of patients, including more male subjects, needs to be conducted. 5. Conclusion
The measurements of resected bones as performed in this study provides a new approach to evaluating the suitability of total knee prostheses. There are two important points to consider: (1) the measurement of diseased bones is more suitable than that of the normal bones since they are the ones which require total knee replacement; (2) the surface of the resected bone making contact with the prosthesis should be considered more important than the superficial position. Therefore, dimensions of the resected surfaces of the OA knees should be considered as an important factor in the design of the tibia1 baseplate. References [l] Freeman M.A.R., Todd R.C., Bamert P., Day W.H.. ICLH arthroplasty of t:he knee: 1968-1977. J Bone and Joint Surg 1978;60B:339-344. [2] Insall JN. Revision of aseptic failed total knee arthroplasty. In: Insall JN, editor. Surgery of the Knee 2nd, ed. Churchill Livingston, New York, 1994:935-57.
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[3] Krackow KA. Prostheses selection. The technique of total knee arthroplasty. C.V. Mosby, St Louis, 1991:49-74. [4] Insall JN. Surgical techniques and instrumentation in total knee arthroplasty. In: Insall JN. Surgery of the knee. 2nd ed. Churchill Livingston, New York, 1994:935-57. [5] Liu H.C.. Review of gross anatomy of the Chinese knee. J Formosan Med Assoc 1984;83:317-325. [6] Mensch J.S., Amstuta H.C.. Knee morphology as a guide to knee replacement. Clin Orthop 1975;112:231-241. [7] Erkman MJ, Walker PS. A study of knee geometry applied to the design of condylar prostheses. Biom Eng 1974; January: 9-14. [B] Seedham BB et al. Dimensions of the knee. Ann Rheum Dis 1972: 31-54. [9] Wang S.W., Feng C.H., Lu H.S.. Original articles: a study of the Chinese knee joint geometry for prostheses design. Chinese Medical Journal 1992;105(3):227-233. IO] Ishinishi T, Ogata K, Nishino I, White SE, Whiteside LA. Comparison of Asian and Caucasian normal and osteoarthritic knees. In: 40th Ann Meeting, Ortho Research Sociey, 1994: 663-4. 1l] Little R.B., Wever H.W., Siu D., Cooke T.D.. A three-dimensional finite element analysis of the upper tibia. J Biomech Eng 1986;108(2):111-119. 121 Schreppers G.J., Sauren A.A., Huson A.. A numerical model of the load transmission in the tibia-femoral contact area. Proceeding of the Institution of Mechanical Engineers. Art H-J of Engineering in Medicine 1990;204( 1):53-59. [13] Walker P.S.. Design of Kinemax total knee replacement, bearing surface. Acta Orthop Belgica 1991;57(supple 2): 108-113. [14] Whiteside L.A., Pafford J.. Load transfer characteristics of a non-cemented total knee arthroplasty. Clin Orthop 1989;239: 168-177. [15] Ward W.G., Johnston K.S., Dorey F.J., Eckardt J.J.. Extramedullary porous coating to prevent diaphyseal osteolysis and radiolucent lines around proximal tibia1 replacements. J Bone and Joint Surg 1993;75-A:976-987,