Trabecular microstructure in the medial condyle of the proximal tibia of patients with knee osteoarthritis

Bone Vol. 17, No. 1 July 1995:27-35 ELSEVIER

Trabecular Microstructure in the Medial Condyle of the Proximal Tibia of Patients with Knee Osteoarthritis L. K A M I B A Y A S H I , 1 U. P. W Y S S , 2 T. D. V. C O O K E , 3 and B. Z E E 4 i Department of Anatomy and Cell Biology, 2 Clinical Mechanics Group, and 4 Clinical Trials Group, Queen's University, Kingston, Canada 3 King Faisal Specialist Hospital, Riyadh, Kingdom of Saudi Arabia

thetic fixation at the bone-implant interface. In knee joint arthroplasty, loosening of the tibial component is one of the major causes of prosthetic failure necessitating revision arthroplastic surgery. 7 A better understanding of the microstructural organization and the biomechanical competence of OA cancellous bone in the proximal tibial plateau may lead to improvements in prosthetic fixation and longevity. Bone volume fraction or apparent bone density is the most common histomorphometric parameter measured and correlated to biomechanical properties. It is a measure of the amount of bone present per unit volume of specimen and it has a significant correlation to mechanical properties such as stiffness and strength. 1.3.9.12 The problem has been that the exact characterization of the mathematical relationship between apparent density and mechanical properties has been variable among different investigators, expressed as a function of linear regressions or power functions, s According to Ciarelli et al.,4 apparent density can only explain 40-80% of the variance between localized regions of cancellous bone. Therefore, other measures of trabecular microstructure may account for some of the differences in mechanical properties found between different localized regions of the same bone with the same bone volume fraction. Measurement of individual trabecular parameters such as trabecular thickness and trabecular number as well as the interaction of trabecular as measured by trabecular separation and trabecular connectivity provides a more detailed description of the overall trabecular architecture than does bone volume fraction alone. In normal bone, increases in bone volume fraction have been positively associated with increases in trabecular number and connectivity with no change in trabecular thickness. 1o In pathological OA bone, it is possible that these microstructural parameters change in a manner that is different from normal bone. In this article, trabecular microstructure of the weight-bearing medial condyle of the tibial plateau of an osteoarthritic group is compared to an age-matched control group to test the hypothesis that in OA of the knee, subchondral sclerosis does not only involve an increase in bone volume fraction but also an alteration in microstructural characteristics such as trabecular thickness, trabecular number, trabecular separation, and trabecular connectivity.

The microstructural characteristics of osteoarthritic subchondral bone in the medial tibial condyle are clearly different from normal age-matched bone. Subchondral sclerosis in osteoarthritis indicates not only an increase in bone volume fraction but also alteration in other microstructural characteristics. Eleven medial tibial condyles were collected from ten subjects during arthroplastic surgery for knee osteoarthritis. They were compared to four medial tibial condyles from four age-matched controls with no history of any bone or joint disorder. Six sections from anterior to posterior and three levels from proximal to distal were evaluated in each medial condyle. Five histomorphometric parameters were measured: bone volume fraction (BVf), trabecular thickness (Tb.Th), trabecular number (Tb.N), trabecular separation (Tb.S), and trabecular connectivity (Tb.C). In general, the osteoarthritic subchondral bone had a higher bone volume fraction than control bone but the microstructure was characterized by fewer, widely spaced, thicker than normal trabecnlae. There were also highly localized regional differences by depth from the articular surface and from anterior to posterior across the medial condyle. These variations in OA subchondral bone microstructure may significantly affect biomechanical competence of bone in a way not predictable by bone volume fraction measurements alone. (Bone 17:27-

35; 1995) Key Words: Osteoarthritis; Knee; Tibia; Subchondral bone; Microstructure; Histomorphometry.

Introduction Primary osteoarthritis (OA) is a progressive disease of unknown etiology that leads to destructive changes in the articular cartilage and underlying subchondral bone within a joint. At the local joint level, extensive remodeling of these tissues may alter the geometry of the articular surfaces and subsequently alter the local contact stresses in that joint. 13"15 In the knee joint, total knee arthroplasty or hemiarthroplasty is one established corrective procedure for knee OA intended to prevent further destruction of articular surfaces and to improve physical function. The clinical interest in understanding the trabecular microstructure at the proximal tibial plateau relates to success of pros-

Materials and Methods Description of Subjects

Address for correspondence and reprints: Lynne Kamibayashi, Ph.D., Department of Physiology, Queen's University, Kingston K7L 3N6, Ontario, Canada. © 1995 by ElsevierScienceInc.

Eleven tibial medial condyles were collected during arthroplastic surgery from ten patients with a mean age of 63.8 years (range 57.0-83.8 years) diagnosed clinically and radiographically as 27

8756-3282/95/$9.50 SSDI 8756-3282(95)00137-3

28

L. Kamibayashi Trabecular micmstructure of the proximal tibia

Bone Vol. 17, No. 1 July 1995:27-35

having primary OA of the knee. Four were female and six were male. The female subjects had a mean weight of 84.1 kg (range 68.0-98.0 kg) and a mean height of 157.6 cm (range 151.0162.5 cm). The male subjects had a mean weight of 80.2 kg (range 66.0-90.0 kg) and a mean height of 166 cm (range 160.0172.0). There was no significant difference in weight and height between the female and male subjects so the data were pooled. Seven of the patients had hemiarthroplasty of the medial compartment of the tibia and three had total condylar replacements. Since one patient had a bilateral operation, both medial condyles from one patient were collected. Table 1 describes the radiographic characteristics o f the OA patient group. Four agematched control medial tibial condyles were collected postmortem from four male subjects with a mean age of 67.6 years (range 59.6-75.6 years) with no history of musculoskeletal problems and with no gross evidence of articular degeneration. Information on the exact weight and height of the control group was not available; however, on inspection these subjects were approximately in the same range of weight and height as the OA group.

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Macroscopic Evaluation of Cartilage Degeneration Gross tissue degeneration patterns including fragmentation and fibrillation of cartilage, eburnation of subchondral bone, and presence of osteophytes are listed in Table 1 for the articular regions of the femur, tibia, and patella during surgery. A detailed grading scheme and map of articular cartilage degeneration patterns was made for each medial tibial condyle prior to sectioning (see Figures 3a and b).

Figure 1. Superior view of the proximal tibial condyles. On the medial condyle, the intersection between the lines of the maximum mediallateral width of the medial condyle and the perpendicular line at 50% the maximum medial-lateral width of the medial condyle parallel to the maximum anterior-posterior depth. Sampled sites are indicated by numbers from 1-6.

Preparation of Bone Specimens Although both femoral and tibial specimens were collected from the medial compartment during surgery, only the tibial specimens were sufficiently large enough to section, fix, and embed in preparation for bone histomorphometric analysis. The area of interest was the central weight-bearing portion of the medial condyle o f the tibia within the boundaries of the medial meniscus as illustrated in Figure 1. Each medial condyle was coronally sectioned into six 5 m m sections (anterior to posterior) parallel to the maximum condylar width mediolaterally using a surgical saw. The bone sections were fixed in 70% ethanol at room temperature for at least 24 h and the solution was upgraded to 100%

ethanol over one week. The sections were placed in 100% acetone for 7-10 days with at least four changes of the acetone bath and gentle agitation. The specimens were then embedded in Epon 812, an epoxy resin, in a series of two impregnations involving 1:1 and 1:2 ratios of acetone to Epon 812, respectively. Bubbles were removed in a vacuum oven (28 lb) for 24 h and then the specimen blocks were placed in a hot air oven at 55°C for another 24 h for final hardening. The specimen blocks were milled on one side using a Reichert Polycut microtome and then glued to acrylic sheets using more Epon 812. Once the block adhered firmly to the acrylic, the second side was milled in preparation for staining.

Table 1. Evaluation of OA patients. The type of surgery performed was a medial hemiarthroplasty or total knee arthroplasty. The hip-knee-ankle angle (HKA angle) was measured for an overall varus or valgus limb alignment. Tissue degeneration patterns and osteophytes were evaluated for the medial and lateral femoral condyles, the medial and lateral tibial condyles, and the patella. Code

Knee

Surg

HKA

Fem med

Fem lat

Tibia med

Tibia lat

Pat med

Pat lat

L01 L02 L03 L04a L04b L05 L06 L07 L08 L09 L10

L R L R L R R L L R R

hemi med hemi reed hemi med total hemi med hemi med hemi med hemi med total total hemi med

- 4 - 11 - 8 - 5 - 10 - 9 +5 - 12 - 10 - 6 - 8

Ye* Yf* Yf* Yf Yf Ye* Ye* Ye* Ye* Ye* Ye

N N N Yf Yf N* N N Yf* Ye* Yf

Ye* Yf* Yf* Ye Ye Ye* Ye* Ye* Ye* Ye* Ye

Yf N N Ye Yf N* N N Ye* Ye Yf

N* N* Yf* Y* Y N* N* N* N* Yf* Yf

N* N* Yf* N* N N* N* N* N* Yf* Yf

L = left leg, R = right leg; Surgery = medial hemi-arthroplasty; total knee arthroplasty; HKA = hip-knee-ankle angle, negative is varus and positive is valgus; Y = yes, degeneration present; N = no gross degeneration present; f = fibrillation and fragmentation of cartilage; e = eburnation of subchondral bone; * = presence of osteophytes.

L. Kamibayashi Trabecular microstructureof the proximal tibia

Bone Vol. 17, No. 1

July 1995:27-35 A modified von Kossa staining technique6'14 was used to demonstrate trabecular matrix by substitution of the calcium ions for the silver ions in silver nitrate. This technique allows contrast of the trabeculae from the surrounding marrow spaces enhancing trabecular edge detection during semiautomatic image analysis.

Measurement Sites of Subchondral Bone In order to evaluate similar locations of different medial condyles, it was essential to choose reproducible points of measurement scaled to fit the individual sizes of tibiae. This was accomplished by measuring the intersection point between the maximum condylar width of the medial condyle (medial-lateral) and the maximum depth of the medial condyle (anteriorposterior). Six sites from anterior to posterior were selected on the central weight-beating portion of the medial tibial condyle at 50% of the maximum transcondylar width as shown in Figure 1. A 5 mm × 5 mm grid was placed parallel and immediately below the chondro-osseous junction. Three levels: 1, 3, and 5 mm below the chondro-osseous junction were evaluated, as indicated in Figure 2. On each medial condyle, three levels and six sections were evaluated for a total of eighteen sites. In occasional cases of extensive subchondral cyst formation or fibrosis of osteoarthritic specimens, some of the levels could not be adequately evaluated.

Image Analysis The histomorphometric analysis was conducted using a semiautomatic image analysis system, MCID (Imaging Research Inc., St. Catharine's, Ontario) with a resolution of 512 x 512 pixels. The image was enhanced by differential color shading of 256 gray levels from 0 (lightest) to 256 (darkest). The specimen was placed on a Northern Light table with maximum illumination around 1450-1480. A 55 Macro Nikon lens with 20 mm and 36 mm extension rings was mounted using an F-C adaptor to a CCD Video Camera Recorder. The lens was focused at about 0.5 m so that the entire 5 mm × 5 mm specimen grid filled the computer screen. Digitization was done directly from the prepared stained bone specimens and the digitized image was stored for additional calculations.

Bone Histomorphometric Parameters The first four histomorphometric parameters were derived from equations outlined by Parfitt et al. 16.17 based on the plate model for cancellous bone. Connectivity was measured directly from the digitized image based on resolution of trabeculae into "struts" or vectors with a length and direction. "Nodes" represented intersection points of adjacent struts. The strut and node terminology was derived from Compston et al. 5

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Figure 2. Coronalview of a sectionindicatingdifferentlevels. Levels 1, 2, and 3 were 1, 3, and 5 mm from the articular surface, respectively.

29

Bone Parameters (1) Bone volume fraction (%). Bone volume fraction (BVf) is a direct index of cancellous bone volume (BV) occupied per total tissue volume (TV). This represents apparent bone density. (2) Trabecular thickness (p,m). Trabecular thickness (Tb.Th) is an estimated index of mean trabecular thickness. (3) Trabecular number (/mm). Trabecular number (Tb.N) is derived from the frequency with which the scanning line will intersect with a structural element of bone averaged over all directions of scanning. (4) Trabecular separation (p,m). Trabecular separation (Tb.S) is derived from the shortest distance between trabecular plates on the assumption that they are parallel. (5) Trabecular connectivity (%). Trabecular connectivity (Tb.C) describes the ratio of the number of trabecular intersections (nodes) to the number of individual trabeculae (struts) within a selected area. Trabecular nodes represent intersection points of trabeculae. No connectivity would be indicated by 0% while high connectivity would be close to 100%.

Statistical Analyses Differences in the five histomorphometric parameters between OA (n = 11) and control subjects (n = 4) in the six sections (anterior to posterior) and three levels (1, 3, 5 mm) were evaluated using an analysis of variance repeated measures model. This model allows for the detection of differences between groups, sections, levels, groups by section, and groups by level. Results were considered significant at p ~< 0•05. Results

Macroscopic Evaluation of Gross Tissue Degeneration Patterns Table 1 lists the gross tissue degeneration patterns documented during surgery. In the femur, all eleven specimens demonstrated cartilage degeneration on the medial condyle with loss of full thickness of cartilage and eburnation in seven of the eleven cases. The lateral femoral condyle was considerably less affected and showed degeneration in five cases, three of which received total condylar replacements• Osteophytes were more often present on the medial femoral margin in eight out of eleven cases than on the lateral femoral margin in three out of eleven cases. In the tibia, the medial condyle exhibited cartilage degeneration in all eleven specimens with loss of full thickness of cartilage and eburnation of varying degrees present in nine of the eleven specimens. Degeneration and loss of cartilage was also present in four lateral tibial condyles, three of which received total condylar replacements. Osteophytes were present on the medial tibial margin in eight out of eleven cases and on the lateral tibial margin in two of eleven cases• There were only slight degenerative changes in the cartilage of the patella; however, osteophytes were present in nine out of eleven cases on both the medial and lateral patellar margins. Figures 3 and 4 describe the cartilage degeneration patterns specifically in the medial tibial condyle. The cartilage degeneration patterns appear to be generally within the regions of the anterior portion of the medial tibial condyle and the external portion of the medial tibial condyle toward the medial collateral ligament. There is less degeneration of articular cartilage in the posterior region and the region internally toward the intercondylar eminence. In Figures 3a and b, the degeneration patterns have been classified as either primarily anterior-type in origin or pri-

30

L. Kamibayashi Trabecular microstructure of the proximal tibia

Bone Vol. 17, No. 1 July 1995:27-35

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Figure 3. Anterior- and external-type cartilage degeneration patterns in the medial tibial condyle in OA. Cartilage degeneration patterns were plotted using the same coordinate system (anterior/posterior; external/internal) irrespective of right or left leg to facilitate comparison. External refers to periphery of the knee toward the medial collateral ligament and internal refers to the direction toward the intercondylar eminence. The cartilage degeneration was graded from Stage I to Stage IV. Stage I represented no articular changes. Stage II represented slight fibrillation and/or fragmentation of articular cartilage. Stage III represented moderate erosion of articular cartilage and Stage IV represented loss of full thickness of cartilage, exposure, and ebumation of subchondral bone. (a) Anterior-type wear. Notice the relative increases in the degree of anterior wear from Subject L03 to Subject L08, respectively. (b) External-type wear. Notice the relative increases in the degree of external wear from Subject L01 to Subject L02, respectively. marily external-type in origin depending on the hypothesized location of initial degeneration. They have been ranked in order of the amount of degeneration. In the more severe cases of degeneration, it becomes more difficult to elucidate the initiation site since the degeneration seems to spread across the entire medial condylar surface covering both the anterior and external sites.

fraction (BVe) and trabecular thickness (Tb.Th). Bone volume fraction in the OA group (mean = 54.282%) was greater than the control group (mean =29.556%). Trabecular thickness in the OA group (mean = 243.233 Ixm) was also greater than the control group (mean = 146.278 Ixm).

Bone Histomorphometry

The comparison by level are pooled data of both the OA and normal groups for level 1 at 1 ram, level 2 at 3 mm, and level 3 at 5 mm below the chondro-osseous junction. There were statistically significant differences (p ~< 0.05) in the comparison by level in all of the parameters. Bone volume fraction at level 1 (mean --- 53.021%) was the highest followed by level 2 (mean = 41.090%) and level 3 (mean = 31.645%) with the least. Trabecular thickness showed a similar trend with level 1 (mean = 223.337 Ixm) having the thickest, level 2 (mean = 202.651 ixm) having intermediate thickness, and level 3 (mean = 158.278 Ixm) having the thinnest trabeculae. For trabecular number, level 1 (mean = 2.103/ram) had fewer than level 2, but level 2 (mean = 2.232/mm) had more than both levels 1 and 3 (mean -- 1.600/mm). For trabecular separation, level 3 (mean

Comparison by group. Table 2 lists the five histomorphometric parameters for the OA and control groups. In a study by Goulet et al., 1° the range of normal values for static bone parameters was studied from the metaphyseal region of different long bones. The normal range for bone volume fraction was 6-36%, trabecular thickness was 100-190 Ixm, trabecular number was 0.61-2.06/mm, and trabecular separation was 320-1670 Ixm. The control values from this study were within the range of normal values with the exception of trabecular number, which was slightly higher than the range given by Goulet et al. lo There were statistically significant differences (p ~< 0.05) between OA and control group of the following parameters in bone volume

Comparison by Level

Bone Vol. 17, No. 1 July 1995:27-35

L. Kamibayashi Trabecular microstructure of the proximal tibia pustenor

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Figure 3. Continued. = 577.592 Ixm) had the greatest separation followed by level 1 (mean = 328.753 p,m) and level 2 (mean = 300.773 Ixm) had the least separation distance. Trabecular connectivity in level 1 (mean --- 47.778%) was less than level 2 (mean = 58.164%) and level 2 was greater than level 3 (mean = 51.846%).

Comparison by Section The comparison by section represents the pooled data of both the OA and control group from sections 1 to 6, respectively, anterior 12to 10" E

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to posterior. In the comparison by section, there were no statistically significant differences in any of the bone parameters, but there were some trends observed. In F i g u r e Sa, bone volume fraction was highest in the middle in sections 3 and 4 and also anteriorly in section 2. In Figure 5b, trabecular thickness was also greatest in the middle in sections 3 and 4, decreasing anteriorly and posteriorly. Trabecular connectivity showed a similar increase in the middle in section 4, decreasing anteriorly and posteriorly, as shown in Figure 5e. In Figure 5c, trabecular number was highest in the middle, sections 2, 3, 4, and 5, decreasing anteriorly and posteriorly. In Figure 5d, trabecular separation was lowest in the middle, sections 3 and 4, increasing anteriorly and posteriorly. In general, the central sections of the medial tibial condyle have the highest values for BVf, Tb.Th, Tb.C., and Tb.N with the lowest Tb.S. Table 2. Comparison by group: control vs. OA. The differences between control group and OA group.

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Figure 4. Anterior-posterior cartilage degeneration patterns of the medial tibial condyle. Stage 1 = no articular changes; Stage 2 = slight fibrillation and/or fragmentation of articular cartilage; Stage 3 = moderate erosion of articular cartilage; Stage 4 = loss of full thickness of articular cartilage; exposure and eburnation of subchondral bone.

BVf (%) Tb.Th (p,m) Tb.N (/mm) Tb.S (~m) Tb.C (co)

Control (mean ± S.E.) 29.556 146.278 2.069 392.032 50.514

± ± ± ± ±

1.482 10.488 0.079 24.919 1.841

Osteoarthritic (mean ± S.E.) 54.282 243.233 1.888 412.713 54.679

± 1.179 ± 8,139 ± 0.064 -+ 20.619 ± 1.460

p ~< 0.05 * * NS NS NS

* = significant difference atp ~< 0.05. NS = no significant difference at p ~< 0.05.

32

L. Kamibayashi Trabecular microstrucmre of the proximal tibia

Bone Vol. 17, No. 1 July 1995:27-35

Table 3. Comparison by level. The differences between levels, 1, 2 and 3. Level 1 (mean - S.E.)

Parameter BVf (%) Tb.Th (p,m) Tb.N (/mm) Tb.S (p,m) Tb.C (%)

53.021 223.337 2.103 328.753 47.778

Level 2 (mean -+ S.E.)

± 1.587 ± 11.098 -+ 0.086 ± 27.008 ± 2.000

41.090 202.651 2.232 300.773 58.164

Level 3 (mean ± S.E.)

± 1.529 --+ 10.761 ± 0.084 _ 26.393 ± 1.917

31.645 158.278 1.600 577.592 51.846

+ ± ± ± ±

p ~< 0.05

1.765 12.467 0.093 29.782 2.148

* * * * *

* = significant difference at p ~< 0.05. NS = no significant difference at p ~< 0.05.

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Bone Vol. 17, No. 1 July 1995:27-35

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L. Kamibayashi Trabecular microstructure of the proximal tibia

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Comparison of Group by Level The group by level analysis compared the OA and control group at levels 1, 2, and 3. There were no statistically significant differences between the OA and control groups when compared by level. Both the OA and control groups demonstrated similar trends by level. In the case of bone volume fraction (Figure 6a), trabecular thickness (Figure 6b), and trabecular connectivity (Figure 6e), the OA group has higher values than the control group at every level. In both groups, bone volume fraction and trabecular thickness are greatest at level I and incrementally decrease at levels 2 and 3, respectively. In Figures 6c and 6e, level 2 showed an increase in trabecular number and trabecular

connectivity relative to level 1 in both the OA and control groups. In Figure 6d, there was a corresponding decrease in trabecular separation distance at level 2. This indicates that there may be architectural changes occurring at level 2 below the subchondral surface that are not necessarily related to changes in bone volume fraction and trabecular thickness.

Comparison of Group by Section The group by section analysis compared the OA and control groups at sections 1-6. There were no statistically significant differences in the group by section comparison. Figures 7a---e

34

L. Kamibayashi Trabecular microstructure of the proximal tibia

Bone Vol. 17, No. 1 July 1995:27-35

show the patterns for both the OA and control group by section. Bone volume fraction, trabecular thickness, and trabecular connectivity were greater in the OA group than the control group across the sections from 1-6, anterior tO posterior, respectively. In all these graphs, the central sections had the highest values for bone volume fraction, trabecular thickness, and trabecular connectivity decreasing anteriorly and posteriorly. Trabecular separation was generally greater in the OA group than the control group with the exceptions of sections 3 and 6. Trabecular separation tended to decrease in the central sections, increasing anteriorly and posteriorly. Trabecular number was less in the OA group compared with the control group except in section 3. Trabecular number was highest in the central sections decreasing anteriorly and posteriorly. A

Discussion

Osteoarthritic wear of the articular cartilage in the medial tibial condyle was located anteriorly and medially toward the external edge of the medial condyle, while less wear occurred posteriorly and internally toward the intercondylar eminence. At the early stages of degeneration it may be possible to characterize the degeneration as anterior-type or external-type initiated in these locations. As the degeneration progresses covering a larger area it becomes more difficult to identify the possible initiation site. When comparing OA and age-matched controls, OA bone has a greater bone volume fraction than control bone. However, it is not only the absolute amount of bone but also the microstructural characteristics of the trabeculae that have been altered. 350

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6

Bone Vol. 17, No. 1 July 1995:27-35

The OA bone has a greater trabecular thickness and trabecular connectivity but decreased numbers of trabeculae with increased trabecular separation. The alterations of these parameters may have a more significant predictive value for implant-bone ingrowth and stability than might be predicted from bone volume fraction alone. In fact, according to Goulet et al.,lo an increase in bone volume fraction in normal bone is usually associated with an increase in trabecular number and trabecular Connectivity. However, we found that in OA bone, there was a decrease in trabecular number and increased trabecular separation. In addition to the overall microstructural differences between normal and OA bone, there were highly localized variations in different regions within of the proximal tibia. Localized variation in bone volume fraction in previous studies of OA tibia2 and in OA femoral heads n has been related to variations in high vs. low weight-beating regions. When our data were analyzed by level, there were significant localized variations by level. In general, level 1 had the highest bone volume fraction and trabecular thickness, followed by incremental decreases in levels 2 and 3, respectively. Similar incremental decreases in bone volume fraction progressing distally down the tibia were measured by Shimizu et a l ) 8 One would have expected corresponding decreases in trabecular number and trabecular connectivity with increased trabecular separation to accompany the incremental decrease in bone volume fraction and trabecular thickness; however, this was not the case. In fact, level 2 showed a focal increase in trabecular number and trabecular connectivity with a decrease in trabecular separation relative to level 1. This was not predicted by the overall incremental decrease in bone volume fraction and trabecular thickness from levels 1-3. The comparison by section of different sections from anterior to posterior indicated an increase in bone volume fraction, trabecular thickness, trabecular number, and trabecular connectivity in the central region of the medial condyle and decreasing bone volume fraction anteriorly and posteriorly in both the control and OA groups. Trabecular separation distances were lowest centrally and increased anteriorly and posteriorly. These changes in bone parameters in the central sections appear to be related to the loss of articular cartilage and increased wear typical of this anterior/middle region in the OA group. In conclusion, OA subchondral bone differs from agematched control bone not only in bone volume fraction but also in other microstructural parameters such as trabecular thickness, number, separation, and connectivity. There are highly localized variations of these microstructural parameters depending on the relative depth from the articular surface and also variations anterior to posterior across the medial tibial condyle. Theseregional variations may significantly affect the biomechanical competence of subchondral OA bone.

Acknowledgments: W e would like to thank Lou Franchi and his staff in the Department of Pathology at Kingston General Hospital for the preparation of the bone specimens. W e are grateful to Dr. Paul Gross in the Department of Surgery and Lloyd Kennedy of the Department of Pathology for allowing us access to their image analysis systems. W e also acknowledge Krisjanis Steins and Pat Costigan for their assistance in computer programming.

L. Kamibayashi Trabecular microstmcture of the proximal tibia

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Date Received: May 16, 1994 Date Revised: March 15, 1995 Date Accepted: March 15, 1995