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Calcified cartilage, subchondral and cancellous bone morphometry within the knee of normal subjects
ELSEVIER
The Knee 3 (1996) 15-22
KNEE
Calcified cartilage, subchondral and cancellous bone morphometry within the knee of normal subjects Barbara Koszyca”, Nicola L. Fazzalari* b, Barrie Vernon-Robertsb ‘Department of Histopathologv, bDiuision of Tissue Pathology,
Women’s and Children’s Hospital, Institute of Medical and Veterinay
72 King William Road, Adelaide, South Australia Science, Frome Road, Adelaide, South Australia
5006, Australia 5000, Australia
Accepted 15 April 1996
Abstract This study examined calcified cartilage and bone structure in the development of age-related cartilage changes in the knee. Image analysis was used to examine the calcified cartilage and bone structure. There was a significant decrease in bone volume fraction with increasing age in the knee except for the patella. The study found the cancellous network of the patella to be markedly different from other regions of the knee. The bone volume of the patella is significantly higher and fails to show a decrease with age. A correlation exists between subchondral bone and both the bone volume and the trabecular thickness of the adjacent cancellous network. A correlation between calcified cartilage thickness and the bone volume and the trabecular thickness of the adjacent cancellous network exists only in the patella. It is proposed that this relationship is an adaptation in dealing with high shear forces in the pate110 femoral joint. Keywords:
Knee; Calcified
cartilage;
Subchondral
bone; Cancellous -
1. Introduction
Cartilage damage is a characteristic feature of osteoarthritis but similar cartilage changes can be seen in ageing joints. Thus before speculating on the pathogenesis of osteoarthritis it is necessary to have a clear understanding of the effects of ageing alone on synovial joints. One of the many hypotheses regarding the development of cartilage changes in osteoarthritis involves the structure and density of cancellous bone. It has been shown that cancellous bone network hals a significant role in the absorption of potentially damaging loading forces [l]. Changes in structure will affect compliance and may thus expose cartilage to potentially damaging forces [l-31. This study was performed in order to determine whether cancellous bone structure has a role in the development of the
*Corresponding author, Tel.: +61 8 2287269; fax: +61 2287204; email: [email protected]. 0968-0160/96/$15.00 PII SO968-0160(96)
8
0 1996 Elsevier Science B.V. All rights reserved 00206-2
bone
age-related cartilage changes seen in the knee, and also whether regional differences in cartilage condition showed any relation to cancellous bone structure. The relationship between cancellous bone and structures at the bone cartilage interface such as the calcified cartilage and the subchondral bone plate was also examined to determine their possible role in the development of cartilage damage. 2. Materials amd methods
A total of 67 knee joints were obtained from 34 individuals, 15 female and 19 male, ranging in age from 18 to 90 years with a mean age of 61.8 f 18.2 years. Excluded from the study were individuals known to have a history of knee disease, to have had trauma or knee surgery, or to be suffering from metabolic bone disease or disseminated malignancy. Histological examination was restricted to the trochlea, patella, medial femoral condyle and medial tibia1 facet. Specimens were removed during autopsy
16
B. Kosqca
et al. /The
by dissecting away the soft tissues, sectioning the femur and tibia at 7-10 cm from the tibiofemoral joint surface and disarticulating the tibiofibular joint. The specimens were immediately disarticulated and placed in 10% neutral formalin. After macroscopic assessment the medial tibia and femur specimens were cut into 5mm thick coronal slices and the trochlea and patella were cut into 5-mm thick sagittal slices using a band saw. The two most central slices were selected. From the most inferior or posterior slice blocks measuring 1.0 by 1.5 cm were cut from the central zone (Fig. 1) using an Isomet slow speed saw (Buehler, Lake Bluff, USA) and were processed without decalcification and embedded in Araldite Epoxy resin (Ciba Geigy, Melbourne, Australia). Five-micrometre thick sections were cut from each block at three levels 200 micrometre apart, using a Jung K microtome (Reichert Jung, Heidelberg, Germany). The sections were mounted and then stained by the Von Kossa technique with an haematoxylin and eosin counter-stain. Histoquantitation was conducted using the Quantimet 520 Image Analyser (Cambridge Instruments, Cambridge, UK) at 10 x objective magnification and examining 20 fields per section. The image was relayed from an Olympus BH2 microscope, via a video camera, to the television screen of the Quantimet. The parameters examined in this study were the percentage bone volume fraction (BV/TV), the surface density of bone in mm2/mm3, (BS/TV), trabecular thickness (Tb.Th) and trabecular spacing (TbSp) both in micrometres. As a result of difficulties in processing, blocks from all four regions of the knee were obtained in only 31 of
LATERAL
MEDIAL
Medial femoral condvle Medial tibia1 plateau Patellar ligament
P”$$ePS Tibia
I
Tissue taken for image analysis
Fig. 1. Anterior aspect of the right knee with the patella forward, indicating the sites from which tissue was taken.
reflected
Knee 3 (1996)
15-22
1
Total cartilage thickness
Osteochondral junction
I
Specimen
length
Fig. 2. Schematic diagram of the bone-cartilage interface. Calcified cartilage thickness = [(Area of calcified cartilage)/ (Specimen length)].
the 34 individuals within the study group. In order to best preserve and define the calcified cartilage layer and the subchondral bone plate, blocks were cut from the more anterior or superior slice in each region using the Isomet slow speed saw, and were decalcified in a mixture of 1% ethylene diamene tetra-acetic acid and 9.5% nitric acid before being processed into paraffin. Five-micrometre sections were cut, mounted and stained by the technique described by Sayers et al. [41. The Quantimet Image Analysing Computer 520 was used to measure the thickness of the calcified cartilage layer and also the thickness of the subchondial plate. The bone volume fraction and the thickness of the subchondral plate were measured to a depth of 1 millimetre below the osteochondral junction, from images at 4 X objective magnification. This method was chosen because of the poorly defined nature of the subchondral plate in the specimens studied, especially those from the tibia. The area of subchondral bone was determined using the image analyser and the mean thickness by dividing the area by the length of the section (Fig. 2). Images at 10 x objective magnification were used to measure the mean thickness of the calcified cartilage layer. The Quantimet was used to determine the area of calcified cartilage within the section, and this value was divided by the length of the section. In cases with multiple tidemarks, the tidemark nearest the articular surface was used in all cases to define the extent of the calcified cartilage. Data analysis was conducted using PC-SAS software (SAS Institute Incorporated, Cary, USA). Before any statistical comparisons were made the nature of the distribution of each data group was determined by
B. Kosqca Table 1 Comparison knees
of bone
histoquantitation
data from
Right N=56 52.3 30.5 5.5 114 267
Age (years) BV/TV(%) BS/TV (mm2/mm3) Tb.Th( pm)
Tb.SpCpm) Values
given
as mean
17.6 6.9 1 .o 21 58
62.5 30.7 5.9 106 249
+ i f * i
Knee 3 (19961 15-22
the left and right
Left N = 68 + + * * k
et ai /The
50
-r
45
t
20
--
15
--
17
P
18.1 7.1 1.0 22 56
< 0.01 NS < 0.05 NS NS
q
.
..
.
10 t
5t
1 SD. NS, not significant.
04
i 0
Table 2 Percentage bone volume the knee examined
10
20
30
40 Age
fraction
(BV/TV)
in the four
Region
Mean
Medial femur Medial tibia Trochlea Patella
28.5 30.8 27.6 35.5
regions
50
80
70
80
90
(years)
of Fig. 3. Regression of bone volume Total study group n = 124, I= 37.46 - 0.12 X Age.
* SD
fraction -0.316,
(BV/TV) on age (years). P< 0.0005, BV/TV=
__-
N = 31 in each region.
Values
given
as mean
+ f + k
5.3 7.2 6.4 6.4
females but not in males alone (Table 3). The correlation was significant in all regions of the knee examined except the patella (Table 3). BV/TV and BS/TV exhibited a positive non-linear correlation in the total study group (Fig. 4) and in males but not females, and in the medial femoral condyle and the medial tibia1 plateau but not elsewhere (Table 4). There was a significant positive correlation between BV/TV and Tb.Th in the total study group (Fig. 5) and this correlation was significant in both males and females and in all regions of the knee examined (Table 5). BS/TV was significantly higher in the trochlea than in either the medial femur (P < 0.05) or medial tibia (P < 0.03) (Table 6). There was no significant correlation of BS/TV with age. Tb.Th was significantly greater in the patella than in any other region except the medial tibia (P < 0.001). Trabeculae were significantly thinner in the trochlea than elsewhere (P < 0.05) (Table 7). Tb.Th and age exhibited a negative correlation, significant in the total study group (Fig. 6), both male and female subgroups and in all regions of the knee (Table 8). Tb.Sp was significantly greater in the medial femoral condyle (P < 0.005) and the medial tibia1
+ SD.
using the Shapiro-Wilk statistic. Depending on lthe distribution, groups were compared using paired and non-paired t-tests or the Wilcoxon rank statistic. Comparisons between more than two subgroups were conducted using a one-way analysis of variiance or multiple Wilcoxon rank tests. Correlations were conducted using the Pearson coefficient of correlation or the Spearman rank coefficient. Linear and non-liniear regressions were conducted as indicated. The critical level of significance in this study was taken to be P < 0.05 for all tests.
3. Results There were no significant gender-related differences, but significant right-left differences existed for age and BS/TV (Table 1). BV/TV was significantly higher in the patella than in any other region of the knee (P < 0.001) (Table 2). A negative correlation between age and BV,/TV was significant in the total study group (Fig. 3), and in Table 3 Correlation
between
age (years)
and percentage
bone volume
Region
n
r
M F MF MT TR PA
74 48 31 31 31 31
-
M. males;
F, females;
MF,
medial
femur;
fraction
(BV/TV)
0.215 0.447 0.397 0.463 0.359 0.188
MT, medial
tibia;
TR, Trochlea;
P
Correlation
NS < 0.01 < 0.01 < 0.01 < 0.05 NS
BV/TV BV/TV BV/TV BV/TV BV/TV BV/TV
PA, Patella.
= = = = = =
36.28 38.30 35.02 40.99 34.65 39.19
-
0.09 0.16 0.11 0.18 0.12 0.06
x x x x x x
Age Age Age Age Age Age
18
B. Koszyca
O-1 0
10
20
30
et al. / The Knee 3 (19961 15-22
I 50
40
04 0
5
10
15
20
BV/TV [%)
Fig. 4. Regression fraction (BV/TV). BS/TV(mm2/mm3)
of total bone surface CBS/TV) on bone volume Total study group n = 124, r = 0.401, P < 0.0001, = 2.14 x BV/TV* 3.
Fig. 5. Regression fraction (BV/TV). Tb.Th (micrometres)
plateau (P < 0.003) than in the patella (Table 9). There was no significant correlation with age. A positive correlation was found between the BV/TV of the cancellous bone measured from the undecalcified sections and the matrix BV/TV of the subchondial plate, measured in the adjacent decalcified sections (Fig. 7). This correlation was significant in the total study group, in both males and females, and regionally in the medial femoral condyle and medial tibia1 plateau, but not elsewhere (Table 10). A significant positive correlation was also found between Tb.Th and the subchondial plate BV/TV in the total study group (Fig. 8) and in both sexes. Regionally, however, the correlation was significant
Table 4 Correlation
between
total
bone surface
(mm2/mm3)
and percentage
25 BWTV f%)
bone volume
fraction
(BV/TV)
r
P
Correlation
M F MF MT TR PA
76 48 31 31 31 31
0.476 0.238 0.405 0.722 0.318 0.233
< 0.0001 NS < 0.05 < 0.0001 NS NS
BS/TV BS/TV BS/TV BS/TV BS/TV BS/TV
Table 5 Correlation
between
MF,
medial
trabecular
femur;
thickness
MT,
medial
(Tb.Th)
tibia;
TR, Trochlea;
in micrometres
and percent
n
r
P
M F MF MT TR PA
74 48 31 31 31 31
0.637 0.826 0.655 0.509 0.810 0.687
< < < < < <
F, females;
MF,
medial
femur;
MT,
medial
tibia;
TR, Trochlea;
= = = = = =
1.6 3.4 1.9 0.6 3.2 2.9
x
x
BV/TV”-4 BV/TV”,2 BV/TVn-3 BV/TV’-’ BV/TV0-2 BV/TV0-2
+ + + + + +
2.2 3.0 2.5 1.6 2.6 2.1
x BV/TV x BV/TV x BV/TV x BV/TV x BV/TV x BV/TV
x x x x
PA, Patella.
Region
M, males;
40
45
I 50
only in the medial femoral condyle and the medial tibia1 plateau (Table 11). A positive correlation between BV/TV of the cancellous bone and calcified cartilage thickness (micrometres) was significant in males only (n = 76, I = 0.278, P < 0.05, Calcified cartilage thickness = 80.4 * 1.8 X BV/TV); and regionally it was significant only in the patella (n = 31, r = 0.483, P < 0.01, Calcified cartilage thickness = 15.3 + 3.8 X BV/TV). A positive correlation between Tb.Th and calcified cartilage thickness was significant only in males (n = 76, r = 0.237, P < 0.05, Calcified cartilage thickness = 88.1 + 0.4 X Tb.Th); and only in the patella (n = 31,
n
F, females;
35
of trabecular thickness (Tb.Th) on bone volume Total study group n = 124, r = 0.714, P < 0.0001, = 32.5 + 2.5 x BV/TV.
Region
M, males;
30
bone
volume
fraction
(BV/TV) Correlation
0.0001 0.0001 0.0001 0.01 0.0001 0.0001 PA, Patella.
Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th
= = = = = =
42.4 17.0 33.9 66.4 21.4 30.4
B. Koszyca Table 6 Total bone surface the knee examined
(BS/TV)
in mm2/mm3
in the four
Region
Mean
F SD
Medial femur Medial tibia Trochlea Patella
5.5 6.0 5.4 5.9
0.9 0.9 1.1 0.9
N = 31 in each region.
Table 7 Trabecular thickness the knee examined
Values
(Tb.Th)
given
+ * i f
et al. /The
regions
Knee 3 (1996)
19
of
60
as mean f SD.
in micrometres
15-22
in the four
regions
of 0
20
40
60
80
100
Age (years)
Region
Mean
Medial femur Medial tibia Trochlea Patella
105 115 94 125
N = 31 in each region.
Values
given
as mean
T = 0.443, P < 0.05, Calcified 38.0 + 0.9 x Tb.Th).
f SD * + f +
Fig. 6. Regression of trabecular thickness (Tb.Th) on age (years) Total study group n = 124, r = -0.456, P < 0.0001, Tb.Th (micrometres) = 144.8 - 0.61 X Age.
20 22 21 25
f SD.
cartilage
thickness =
4. Discussion
Bone loss with increasing age is a well known phenomenon in the head of the femur [5], the iliac crest [6-91, and the lumbar vertebrae [lo]. The current study has shown that there is also a significant decrease in BV/TV in the knee with increasing age, a decrease which is significant in all regions of the knee examined except for the patella. In the medial tibia, the medial femur and the trochlea there was no difference between the slopes of the regression lines, nor in the intercepts. Thus mineralised bone is lost from these regions at a similar rate, and the loss is unaffected by regional differences in function. Only 9% of the variance in BV/TV within the total study group can be accounted for by age. In the female subgroup this figure rose to nearly 20%, reflecting the
Table 8 Correlation
between
age (years)
Region
n
M F MF MT TR PA
76 48 31 31 31 31
M, males;
F, females;
MF,
medial
and trabecular
thickness
(Tb.Th)
profound influence of age on mineralised bone in females, presumably as a result of the effect of the menopause on bone volume. Age-related bone loss is generally accepted to be the result of the loss of entire trabeculae from the cancellous network [7,8,111. This is reflected by an increase in Th.Sp, as changes in Tb.Th alone are rarely sufficient to be reflected by changes in the Th.Sp [5]. However, in contrast to the femoral head [5], and iliac crest [8,11], there was no significant change in Tb.Sp in the knee with increasing age, therefore bone loss in the knee appears to be the result of trabecular thinning alone. This decrease in Tb.Th was seen in all regions of the knee examined, including the patella, despite the fact that the patella showed no significant decrease in BV/TV with age. The rate of trabecular thinning with age did not appear to be significantly different in the four regions examined and there was no significant sex-related difference. This is similar to the different regions within the femoral head [5] where there were also no significant regional differences in the rate of change in Tb.Th associated with changes in BV/TV.
in micrometres P
femur;
MT,
0.450 0.467 0.585 0.667 0.380 0.442
medial
tibia;
< < < < < < TIP, Trochlea;
Correlation 0.0001 0.001 0.001 0.0001 0.001 0.05
PA, Patella.
Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th
= = = = = =
147.6 140.5 141.1 161.4 118.2 158.6
-
0.62 0.58 0.63 0.79 0.42 0.58
x X X X X X
Age Age Age Age Age Age
20
B. Koszyca
Table 9 Trabecular spacing the knee examined
et al. / The Knee 3 (1996) 100
(Tb.Sp)
in micrometres
in the four
Region
Mean
Medial femur Medial tibia Trochlea Patella
267 250 251 231
N = 31 in each region.
Values
given
of
T
+ SD * i i f
as mean
regions
IS-22
49 73 47 47
1 SD.
100 T
04
.
t
0 l
.
50
100
.
150
I 200
Tb.Th (microns)
Fig. 8. Regression of subchondial bone volume fraction (SCB) on trabecular thickness (Tb.Th) in micrometres. Total study group n = 124, r = 0.398, P < 0.0001, SCP = 36.1 + 0.2 x Tb.Th.
04 0
10
20
30 q Vnv
I 50
40
(‘h)
Fig. 7. Regression of subchondral bone volume fraction (SCB) on age (years). Total study group n = 124, r = 0.372, P < 0.0001. SCB = 37.9 + 0.7 x BV /TV.
The pattern of bone loss --- by trabecular thinning rather than by trabecular loss - is unusual and may be because continued intermittent loading is a potent stimulator of bone formation [12]. In these areas with continued loading there may be a strong stimulus to conserve an intact trabecular network and therefore no trabeculae will be lost with increasing age. There is some support for this hypothesis in a study of the femoral head [51, where the loss of trabeculae as reflected by an increase in TbSp, occurred far earlier in the less stressed principal tensile region when compared to the weight bearing principal compressive region. All areas examined within the current study can be considered to have been taken from the ‘prinTable 10 Correlation @V/TV)
between
the percentage
bone volume
fraction
cipal compressive’ regions of the patellofemoral and tibiofemoral joints. This may explain the absence of any significant difference in BV/TV between the sexes, since in the principal compressive region of the femoral head no such difference in BV/TV between males and females existed [5]. BS/TV showed no variation with age, in contrast to previous studies of the iliac crest [81 and the femoral head [3] where a significant decrease with age was seen. In one study of the iliac crest there was a decrease in BS/TV only after the age of 60 [13]. This discrepancy of the effect of age on BS/TV, when compared to other regions of the skeleton, may be related to the fact that few trabeculae are completely lost from the cancellous network during age-related bone loss within the knee. The observation that the BV/TV of the subchondral plate correlated strongly with both the BV/TV of cancellous bone and Tb.Th is similar to the result obtained by Noble and Mexander [14] in the tibia1 plateau. In the present study the correlation was significant only in the medial tibia1 plateau and the medial femoral condyle. Thus the hypothesis of strong pillars being necessary to uphold a thicker subchondral plate [14] appears to be valid only for regions of
of the subchondral
plate
(SCB)
and the bone
volume
Region
n
i-
P
Correlation
M F MF MT TR PA
16 48 31 31 31 31
0.248 0.577 0.671 0.405 - 0.047 0.329
< 0.05 < 0.0001 < 0.0001 < 0.05 NS NS
SCB SCB SCB SCB SCB SCB
M, males;
F, females;
MF.
medial
femur;
MT, medial
tibia;
TR, Trochlea;
PA, Patella.
= = = = = =
fraction
of cancellous
46.2 + 0.5 x BV/TV 22.4 + 1.3 x BV/TV 0.5 + 1.7 x BV/TV 52.5 + 0.6 x BV/TV 60.3 - 0.01 x BV/TV 50.5 + 0.5 x BV/TV
bone
B. Kosiyca Table 11 Correlation
between
the percentage
bone
volume
fraction
et al. / The Knee 3 (1996)
18-22
of the subchondial
(SCB)
plate
21
and trabecular
thickness
Region
n
r
P
Correlation
M F MF MT TR PA
76 48 31 31 31 31
0.223 0.676 0.630 0.369 0.109 0.253
< 0.05 < 0.0001 < 0.0001 < 0.05 NS NS
SCB SCB SCB SCB SCB SCB
M, males;
F, females;
MF,
medial
femur;
MT,
medial
tibia;
TR, Trochlea;
the knee directly involved in weight bearing. The thinning of the subchondral plate as cancellous BV/TV decreases is more rapid in females than in males (P < 0.01); and whilst cancellous BV/TV accounts for only 6% of the variance in subchondral plate BV/TV in males, the figure rises toI 33% in females. This may render women more susceptible to fracture as a result of age-related bone loss. The marked density of the tibia1 subchondral Iplate appears related to its mechanically weak concave surface, where loading acts to separate the constituent elements [15]. Furthermore, as BV/TV decreases in the medial tibia with increasing age, it appears the loss of mineralised bone is mainly from the cancellous bone and not from the subchondral plate where mineralised bone remains to maintain the structural integrity of the concave medial tibia1 plateau [151. A number of hypotheses have been put forward regarding the role of calcified cartilage in synovial joints. It has been suggested that it represents a zone of transitional stiffness, decreasing the stress concentration at the bone-cartilage interface [16,17]. It has been proposed to bond cartilage and bone firmly together by firmly anchoring the collagen fibres of hyaline cartilage [15]. The interdigitation of the calcified layer and the underlying bone has also been proposed to have a role in the conversion of shear stresses to compressive forces [181. The presence of a significant correlation between calcified cartilage thickness and both BV/TV and TbTh may be the result of the fact that as bone becomes more dense, with thicker trabeculae, it will become less compliant [l] and will then require a greater zone of transitional stiffness to minimise cartilage damage. If this is so it is not clear why this relationship is significant only in males but this may be related to the more profound effect of age on bone volume in females, masking any relationship with the calcified layer. Despite not being involved in direct weight bearing the patella has a much denser trabecular network than the other regions of the knee examined. This unusual feature may be the result of the considerable forces to which the patella is exposed, tethered at one end to the tibia and subject to the tensile forfces of the
= = = = = =
(Tb.Th)
in micrometres
47.3 + 0.1 x Tb.Th 16.1 + 0.4 x Tb.Th 4.8 + 0.4 x Tb.Th 50.8 + 0.2 x Tb.Th 53.1 + 0.05 x Tb.Th 54.6 + 0.1 x Tb.Th
PA, Patella.
quadriceps femoris. The dense bone of the patella will render the cartilage susceptible to potentially damaging loading forces [l] and this may explain the extensive cartilage damage typically seen in the patella [19,20]. Although cancellous bone is important in attenuating loading and weight bearing forces, the patella is not directly involved in weight bearing and, therefore, it cannot be assumed that the cancellous bone in the patella has the same importance in absorbing potentially damaging forces. The patella moves across the medial femoral condyle, the trochlea and the subsynovial fat pad [21] and thus would appear to be more exposed to shear forces and the role of cancellous bone in dealing with such forces is not clear. That there is a relationship between bone and cartilage condition in the patella is supported by the work of E&stein [22] on cartilage damage in the patellas of young adults; cartilage damage on the lateral facet of the patella was associated with a high degree of bone mineralisation and was more likely to progress. In contrast lesions on the medial aspect of the patella were associated with medium to low degrees of bone mineralisation and were less likely to progress. The medial lesions were proposed to be the result of shear stresses. The role of shear forces in the development of cartilage damage is also unclear; synovial joints are almost completely frictionless systems [3] but it seems probable that shearing forces may have a role in the progression of such damage. Calcified cartilage has been proposed to play a role in a joint’s ability to deal with shear forces by anchoring the collagen fibres of the hyaline layer and by providing a transitional zone of stiffness [16,17]. However, it has been shown that increased calcified cartilage thickness is associated with increased shear stress levels in the deepest layers of the articular cartilage [23]. If this is correct it would appear that in the patella as bone becomes denser and calcified cartilage becomes thicker, cartilage is rendered more susceptible to damage from both loading and shear forces. This current study has determined the nature of cancellous bone and bone loss in the knee. The exceptional nature of the cancellous bone of the patella
22
B. Kosqca
et al. /The
has also been described and a possible role for the development of age-related cartilage damage forward. The nature of the relationship between cellous bone, calcified cartilage and cartilage has been discussed.
it in put canalso
Acknowledgements The authors’ wish to thank Mrs B Manthey for her technical advice, Mr I Parkinson for his help in the development of the Quantimet program and Mr Tim Rogers for his technical assistance. References [ll
Radin EL, Paul IL. Does cartilage compliance reduce skeletal impact loads? The relative force attenuating properties of articular cartilage, synovial fluid, periarticular soft tissues and bone. Art/r Rheum 1970; 13: 1399144. @I Radin EL, Paul IL, Lowy M. A comparison of the dynannc force transmitting properties of subchondral bone and articular cartilage. J Bone Joint Surg [Am] 1970; 52: 444-456. under [31 Radin EL, Paul IL, Pollock D. Animal joint behaviour excessive loading. Nature 1970; 26: 554-555. G, Bentley G. The demonstration of [41 Sayers DCJ, Volpin bone and cartilage remodelling using Alcian Blue and haematoxylin. Stain Technol 1957; 63: 59-63. NL, Darracott J, Vernon-Roberts B. A quantitative [51 Fazzalari description of selected stress regions of cancellous bone in the head of the femur using automatic image analysis. Metab Bone Dis Rel Res 1983; 5: 119-125. DH, Courpron P. Hupscher EA, ClerI61 Birkenhager-Frenkel monts E, Coutimho MF, Schmitz PIM, Meunier PJ. Age related changes in cancellous bone structure. A two dimensional study in the transiliac and iliac crest biopsy sites. Bone Miner 1988; 4: 197-216. structural analysis of (71 Merz WA, Schenk RK. Quantitative human cancellous bone. Acta Anat 1970; 75: 54-66. M. El Parfitt AM, Mathews CHE, Villanueva, AR, Kleerekoper Relationships between surface volume and thickness of iliac trabecular bone in ageing and osteoporosis - implications for the microanatomic and cellular mechanisms of bone loss. J Clin Invest 1983; 72: 1396-1409. JE, Webb A, Tighe JR. Histomorphomet191 Vedi S, Compston
free
3 (1996)
15-22
ric analysis of bone biopsies from the iliac crest of normal British subjects. Metab Bone Dis Rel Res 1952; 4: 231-236. R. Quantrtative histo[lOI Dunnill MS, Anderson JA, Whitehead logical studies on age changes in bone. J Path Bacterial 1967; 94: 275-291. Aaron JE, Makins NB, Sagreiya K. The microanatomy of [ill trabecular bone loss in normal ageing men and women. C/in Orthop 1987; 215: 260-271. influences in bone re[I21 Chamay A, Tschantz P. Mechanical modelling. Experimental research on Wolffs Law. J Biomech 1972;5:173-180. HH, Meyer W, Sherman D, Massry SG. Quantita[131 Malluche tive bone histology in 84 normal American subjects. Calc Tiss Res 1982; 34: 449-455. K. Studies of subchondial bone density [141 Noble J, Alexander and its significance. J Bone Joint Sq [Am/ 1985; 67: 295-302. DO, Fiechtner JJ. Roman arches, human [151 Sims PA, Graney joints and disease: differences between the convex and concave sides of joints. Arth Rheum 1980; 23: 1308-1311. [I61 Green WT, Martin GN, Eanes ED, Sokoloff L. Microradiographic study of the calcified layer of articular cartilage. Arch Path01 1970; 90: 151-158. M, Schulte E, Putz R. The thickness of the I171 Muller-Gerbl calcified layer of articular cartilage: a function of the load supported. J Anat 1987; 154: 103-l 11. bone in the [181 Radin EL, Rose RM. Role of subchondral initiation and progression of cartilage damage. C/in Orthop 1986; 213: 34-40. GA, Waine H, Bauer W. Changes in the knee joint [191 Bennett at various ages with particular reference to the nature and development of degenerative joint disease. New York: Commonwealth Fund, 1949. of the patellar articuDO1 Meachim G. Age related degeneration lar cartilage. J Anat 1982; 134: 365-371. J, Hungerford DS, Zindel M. Patellofemoral [211 Goodfellow joint mechanics and pathology. 1 Functional anatomy of the patellofemoral joint. J Bone Joint Sq [Br] 1976; 49: 175-181. F, Putz R, Muller-Gerbl M, Steinlechner M, WI Eckstein Benedetto KP. Cartilage degeneration of human patellae and its relationship to the mineralisation of the underlying bone: a key to the understanding of chondromalacia patellae and femoropatellar osteoarthrosis? Surg Radio1 Anat 1993; 15: 279-286. P.31 Anderson DD, Brown TD, Radin EL. The influence of basal cartilage calcification on dynamic juxtaarticular stress transmission. Clin Orthopaedics 1993; 286: 298-307.
The Knee 3 (1996) 15-22
KNEE
Calcified cartilage, subchondral and cancellous bone morphometry within the knee of normal subjects Barbara Koszyca”, Nicola L. Fazzalari* b, Barrie Vernon-Robertsb ‘Department of Histopathologv, bDiuision of Tissue Pathology,
Women’s and Children’s Hospital, Institute of Medical and Veterinay
72 King William Road, Adelaide, South Australia Science, Frome Road, Adelaide, South Australia
5006, Australia 5000, Australia
Accepted 15 April 1996
Abstract This study examined calcified cartilage and bone structure in the development of age-related cartilage changes in the knee. Image analysis was used to examine the calcified cartilage and bone structure. There was a significant decrease in bone volume fraction with increasing age in the knee except for the patella. The study found the cancellous network of the patella to be markedly different from other regions of the knee. The bone volume of the patella is significantly higher and fails to show a decrease with age. A correlation exists between subchondral bone and both the bone volume and the trabecular thickness of the adjacent cancellous network. A correlation between calcified cartilage thickness and the bone volume and the trabecular thickness of the adjacent cancellous network exists only in the patella. It is proposed that this relationship is an adaptation in dealing with high shear forces in the pate110 femoral joint. Keywords:
Knee; Calcified
cartilage;
Subchondral
bone; Cancellous -
1. Introduction
Cartilage damage is a characteristic feature of osteoarthritis but similar cartilage changes can be seen in ageing joints. Thus before speculating on the pathogenesis of osteoarthritis it is necessary to have a clear understanding of the effects of ageing alone on synovial joints. One of the many hypotheses regarding the development of cartilage changes in osteoarthritis involves the structure and density of cancellous bone. It has been shown that cancellous bone network hals a significant role in the absorption of potentially damaging loading forces [l]. Changes in structure will affect compliance and may thus expose cartilage to potentially damaging forces [l-31. This study was performed in order to determine whether cancellous bone structure has a role in the development of the
*Corresponding author, Tel.: +61 8 2287269; fax: +61 2287204; email: [email protected]. 0968-0160/96/$15.00 PII SO968-0160(96)
8
0 1996 Elsevier Science B.V. All rights reserved 00206-2
bone
age-related cartilage changes seen in the knee, and also whether regional differences in cartilage condition showed any relation to cancellous bone structure. The relationship between cancellous bone and structures at the bone cartilage interface such as the calcified cartilage and the subchondral bone plate was also examined to determine their possible role in the development of cartilage damage. 2. Materials amd methods
A total of 67 knee joints were obtained from 34 individuals, 15 female and 19 male, ranging in age from 18 to 90 years with a mean age of 61.8 f 18.2 years. Excluded from the study were individuals known to have a history of knee disease, to have had trauma or knee surgery, or to be suffering from metabolic bone disease or disseminated malignancy. Histological examination was restricted to the trochlea, patella, medial femoral condyle and medial tibia1 facet. Specimens were removed during autopsy
16
B. Kosqca
et al. /The
by dissecting away the soft tissues, sectioning the femur and tibia at 7-10 cm from the tibiofemoral joint surface and disarticulating the tibiofibular joint. The specimens were immediately disarticulated and placed in 10% neutral formalin. After macroscopic assessment the medial tibia and femur specimens were cut into 5mm thick coronal slices and the trochlea and patella were cut into 5-mm thick sagittal slices using a band saw. The two most central slices were selected. From the most inferior or posterior slice blocks measuring 1.0 by 1.5 cm were cut from the central zone (Fig. 1) using an Isomet slow speed saw (Buehler, Lake Bluff, USA) and were processed without decalcification and embedded in Araldite Epoxy resin (Ciba Geigy, Melbourne, Australia). Five-micrometre thick sections were cut from each block at three levels 200 micrometre apart, using a Jung K microtome (Reichert Jung, Heidelberg, Germany). The sections were mounted and then stained by the Von Kossa technique with an haematoxylin and eosin counter-stain. Histoquantitation was conducted using the Quantimet 520 Image Analyser (Cambridge Instruments, Cambridge, UK) at 10 x objective magnification and examining 20 fields per section. The image was relayed from an Olympus BH2 microscope, via a video camera, to the television screen of the Quantimet. The parameters examined in this study were the percentage bone volume fraction (BV/TV), the surface density of bone in mm2/mm3, (BS/TV), trabecular thickness (Tb.Th) and trabecular spacing (TbSp) both in micrometres. As a result of difficulties in processing, blocks from all four regions of the knee were obtained in only 31 of
LATERAL
MEDIAL
Medial femoral condvle Medial tibia1 plateau Patellar ligament
P”$$ePS Tibia
I
Tissue taken for image analysis
Fig. 1. Anterior aspect of the right knee with the patella forward, indicating the sites from which tissue was taken.
reflected
Knee 3 (1996)
15-22
1
Total cartilage thickness
Osteochondral junction
I
Specimen
length
Fig. 2. Schematic diagram of the bone-cartilage interface. Calcified cartilage thickness = [(Area of calcified cartilage)/ (Specimen length)].
the 34 individuals within the study group. In order to best preserve and define the calcified cartilage layer and the subchondral bone plate, blocks were cut from the more anterior or superior slice in each region using the Isomet slow speed saw, and were decalcified in a mixture of 1% ethylene diamene tetra-acetic acid and 9.5% nitric acid before being processed into paraffin. Five-micrometre sections were cut, mounted and stained by the technique described by Sayers et al. [41. The Quantimet Image Analysing Computer 520 was used to measure the thickness of the calcified cartilage layer and also the thickness of the subchondial plate. The bone volume fraction and the thickness of the subchondral plate were measured to a depth of 1 millimetre below the osteochondral junction, from images at 4 X objective magnification. This method was chosen because of the poorly defined nature of the subchondral plate in the specimens studied, especially those from the tibia. The area of subchondral bone was determined using the image analyser and the mean thickness by dividing the area by the length of the section (Fig. 2). Images at 10 x objective magnification were used to measure the mean thickness of the calcified cartilage layer. The Quantimet was used to determine the area of calcified cartilage within the section, and this value was divided by the length of the section. In cases with multiple tidemarks, the tidemark nearest the articular surface was used in all cases to define the extent of the calcified cartilage. Data analysis was conducted using PC-SAS software (SAS Institute Incorporated, Cary, USA). Before any statistical comparisons were made the nature of the distribution of each data group was determined by
B. Kosqca Table 1 Comparison knees
of bone
histoquantitation
data from
Right N=56 52.3 30.5 5.5 114 267
Age (years) BV/TV(%) BS/TV (mm2/mm3) Tb.Th( pm)
Tb.SpCpm) Values
given
as mean
17.6 6.9 1 .o 21 58
62.5 30.7 5.9 106 249
+ i f * i
Knee 3 (19961 15-22
the left and right
Left N = 68 + + * * k
et ai /The
50
-r
45
t
20
--
15
--
17
P
18.1 7.1 1.0 22 56
< 0.01 NS < 0.05 NS NS
q
.
..
.
10 t
5t
1 SD. NS, not significant.
04
i 0
Table 2 Percentage bone volume the knee examined
10
20
30
40 Age
fraction
(BV/TV)
in the four
Region
Mean
Medial femur Medial tibia Trochlea Patella
28.5 30.8 27.6 35.5
regions
50
80
70
80
90
(years)
of Fig. 3. Regression of bone volume Total study group n = 124, I= 37.46 - 0.12 X Age.
* SD
fraction -0.316,
(BV/TV) on age (years). P< 0.0005, BV/TV=
__-
N = 31 in each region.
Values
given
as mean
+ f + k
5.3 7.2 6.4 6.4
females but not in males alone (Table 3). The correlation was significant in all regions of the knee examined except the patella (Table 3). BV/TV and BS/TV exhibited a positive non-linear correlation in the total study group (Fig. 4) and in males but not females, and in the medial femoral condyle and the medial tibia1 plateau but not elsewhere (Table 4). There was a significant positive correlation between BV/TV and Tb.Th in the total study group (Fig. 5) and this correlation was significant in both males and females and in all regions of the knee examined (Table 5). BS/TV was significantly higher in the trochlea than in either the medial femur (P < 0.05) or medial tibia (P < 0.03) (Table 6). There was no significant correlation of BS/TV with age. Tb.Th was significantly greater in the patella than in any other region except the medial tibia (P < 0.001). Trabeculae were significantly thinner in the trochlea than elsewhere (P < 0.05) (Table 7). Tb.Th and age exhibited a negative correlation, significant in the total study group (Fig. 6), both male and female subgroups and in all regions of the knee (Table 8). Tb.Sp was significantly greater in the medial femoral condyle (P < 0.005) and the medial tibia1
+ SD.
using the Shapiro-Wilk statistic. Depending on lthe distribution, groups were compared using paired and non-paired t-tests or the Wilcoxon rank statistic. Comparisons between more than two subgroups were conducted using a one-way analysis of variiance or multiple Wilcoxon rank tests. Correlations were conducted using the Pearson coefficient of correlation or the Spearman rank coefficient. Linear and non-liniear regressions were conducted as indicated. The critical level of significance in this study was taken to be P < 0.05 for all tests.
3. Results There were no significant gender-related differences, but significant right-left differences existed for age and BS/TV (Table 1). BV/TV was significantly higher in the patella than in any other region of the knee (P < 0.001) (Table 2). A negative correlation between age and BV,/TV was significant in the total study group (Fig. 3), and in Table 3 Correlation
between
age (years)
and percentage
bone volume
Region
n
r
M F MF MT TR PA
74 48 31 31 31 31
-
M. males;
F, females;
MF,
medial
femur;
fraction
(BV/TV)
0.215 0.447 0.397 0.463 0.359 0.188
MT, medial
tibia;
TR, Trochlea;
P
Correlation
NS < 0.01 < 0.01 < 0.01 < 0.05 NS
BV/TV BV/TV BV/TV BV/TV BV/TV BV/TV
PA, Patella.
= = = = = =
36.28 38.30 35.02 40.99 34.65 39.19
-
0.09 0.16 0.11 0.18 0.12 0.06
x x x x x x
Age Age Age Age Age Age
18
B. Koszyca
O-1 0
10
20
30
et al. / The Knee 3 (19961 15-22
I 50
40
04 0
5
10
15
20
BV/TV [%)
Fig. 4. Regression fraction (BV/TV). BS/TV(mm2/mm3)
of total bone surface CBS/TV) on bone volume Total study group n = 124, r = 0.401, P < 0.0001, = 2.14 x BV/TV* 3.
Fig. 5. Regression fraction (BV/TV). Tb.Th (micrometres)
plateau (P < 0.003) than in the patella (Table 9). There was no significant correlation with age. A positive correlation was found between the BV/TV of the cancellous bone measured from the undecalcified sections and the matrix BV/TV of the subchondial plate, measured in the adjacent decalcified sections (Fig. 7). This correlation was significant in the total study group, in both males and females, and regionally in the medial femoral condyle and medial tibia1 plateau, but not elsewhere (Table 10). A significant positive correlation was also found between Tb.Th and the subchondial plate BV/TV in the total study group (Fig. 8) and in both sexes. Regionally, however, the correlation was significant
Table 4 Correlation
between
total
bone surface
(mm2/mm3)
and percentage
25 BWTV f%)
bone volume
fraction
(BV/TV)
r
P
Correlation
M F MF MT TR PA
76 48 31 31 31 31
0.476 0.238 0.405 0.722 0.318 0.233
< 0.0001 NS < 0.05 < 0.0001 NS NS
BS/TV BS/TV BS/TV BS/TV BS/TV BS/TV
Table 5 Correlation
between
MF,
medial
trabecular
femur;
thickness
MT,
medial
(Tb.Th)
tibia;
TR, Trochlea;
in micrometres
and percent
n
r
P
M F MF MT TR PA
74 48 31 31 31 31
0.637 0.826 0.655 0.509 0.810 0.687
< < < < < <
F, females;
MF,
medial
femur;
MT,
medial
tibia;
TR, Trochlea;
= = = = = =
1.6 3.4 1.9 0.6 3.2 2.9
x
x
BV/TV”-4 BV/TV”,2 BV/TVn-3 BV/TV’-’ BV/TV0-2 BV/TV0-2
+ + + + + +
2.2 3.0 2.5 1.6 2.6 2.1
x BV/TV x BV/TV x BV/TV x BV/TV x BV/TV x BV/TV
x x x x
PA, Patella.
Region
M, males;
40
45
I 50
only in the medial femoral condyle and the medial tibia1 plateau (Table 11). A positive correlation between BV/TV of the cancellous bone and calcified cartilage thickness (micrometres) was significant in males only (n = 76, I = 0.278, P < 0.05, Calcified cartilage thickness = 80.4 * 1.8 X BV/TV); and regionally it was significant only in the patella (n = 31, r = 0.483, P < 0.01, Calcified cartilage thickness = 15.3 + 3.8 X BV/TV). A positive correlation between Tb.Th and calcified cartilage thickness was significant only in males (n = 76, r = 0.237, P < 0.05, Calcified cartilage thickness = 88.1 + 0.4 X Tb.Th); and only in the patella (n = 31,
n
F, females;
35
of trabecular thickness (Tb.Th) on bone volume Total study group n = 124, r = 0.714, P < 0.0001, = 32.5 + 2.5 x BV/TV.
Region
M, males;
30
bone
volume
fraction
(BV/TV) Correlation
0.0001 0.0001 0.0001 0.01 0.0001 0.0001 PA, Patella.
Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th
= = = = = =
42.4 17.0 33.9 66.4 21.4 30.4
B. Koszyca Table 6 Total bone surface the knee examined
(BS/TV)
in mm2/mm3
in the four
Region
Mean
F SD
Medial femur Medial tibia Trochlea Patella
5.5 6.0 5.4 5.9
0.9 0.9 1.1 0.9
N = 31 in each region.
Table 7 Trabecular thickness the knee examined
Values
(Tb.Th)
given
+ * i f
et al. /The
regions
Knee 3 (1996)
19
of
60
as mean f SD.
in micrometres
15-22
in the four
regions
of 0
20
40
60
80
100
Age (years)
Region
Mean
Medial femur Medial tibia Trochlea Patella
105 115 94 125
N = 31 in each region.
Values
given
as mean
T = 0.443, P < 0.05, Calcified 38.0 + 0.9 x Tb.Th).
f SD * + f +
Fig. 6. Regression of trabecular thickness (Tb.Th) on age (years) Total study group n = 124, r = -0.456, P < 0.0001, Tb.Th (micrometres) = 144.8 - 0.61 X Age.
20 22 21 25
f SD.
cartilage
thickness =
4. Discussion
Bone loss with increasing age is a well known phenomenon in the head of the femur [5], the iliac crest [6-91, and the lumbar vertebrae [lo]. The current study has shown that there is also a significant decrease in BV/TV in the knee with increasing age, a decrease which is significant in all regions of the knee examined except for the patella. In the medial tibia, the medial femur and the trochlea there was no difference between the slopes of the regression lines, nor in the intercepts. Thus mineralised bone is lost from these regions at a similar rate, and the loss is unaffected by regional differences in function. Only 9% of the variance in BV/TV within the total study group can be accounted for by age. In the female subgroup this figure rose to nearly 20%, reflecting the
Table 8 Correlation
between
age (years)
Region
n
M F MF MT TR PA
76 48 31 31 31 31
M, males;
F, females;
MF,
medial
and trabecular
thickness
(Tb.Th)
profound influence of age on mineralised bone in females, presumably as a result of the effect of the menopause on bone volume. Age-related bone loss is generally accepted to be the result of the loss of entire trabeculae from the cancellous network [7,8,111. This is reflected by an increase in Th.Sp, as changes in Tb.Th alone are rarely sufficient to be reflected by changes in the Th.Sp [5]. However, in contrast to the femoral head [5], and iliac crest [8,11], there was no significant change in Tb.Sp in the knee with increasing age, therefore bone loss in the knee appears to be the result of trabecular thinning alone. This decrease in Tb.Th was seen in all regions of the knee examined, including the patella, despite the fact that the patella showed no significant decrease in BV/TV with age. The rate of trabecular thinning with age did not appear to be significantly different in the four regions examined and there was no significant sex-related difference. This is similar to the different regions within the femoral head [5] where there were also no significant regional differences in the rate of change in Tb.Th associated with changes in BV/TV.
in micrometres P
femur;
MT,
0.450 0.467 0.585 0.667 0.380 0.442
medial
tibia;
< < < < < < TIP, Trochlea;
Correlation 0.0001 0.001 0.001 0.0001 0.001 0.05
PA, Patella.
Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th Tb.Th
= = = = = =
147.6 140.5 141.1 161.4 118.2 158.6
-
0.62 0.58 0.63 0.79 0.42 0.58
x X X X X X
Age Age Age Age Age Age
20
B. Koszyca
Table 9 Trabecular spacing the knee examined
et al. / The Knee 3 (1996) 100
(Tb.Sp)
in micrometres
in the four
Region
Mean
Medial femur Medial tibia Trochlea Patella
267 250 251 231
N = 31 in each region.
Values
given
of
T
+ SD * i i f
as mean
regions
IS-22
49 73 47 47
1 SD.
100 T
04
.
t
0 l
.
50
100
.
150
I 200
Tb.Th (microns)
Fig. 8. Regression of subchondial bone volume fraction (SCB) on trabecular thickness (Tb.Th) in micrometres. Total study group n = 124, r = 0.398, P < 0.0001, SCP = 36.1 + 0.2 x Tb.Th.
04 0
10
20
30 q Vnv
I 50
40
(‘h)
Fig. 7. Regression of subchondral bone volume fraction (SCB) on age (years). Total study group n = 124, r = 0.372, P < 0.0001. SCB = 37.9 + 0.7 x BV /TV.
The pattern of bone loss --- by trabecular thinning rather than by trabecular loss - is unusual and may be because continued intermittent loading is a potent stimulator of bone formation [12]. In these areas with continued loading there may be a strong stimulus to conserve an intact trabecular network and therefore no trabeculae will be lost with increasing age. There is some support for this hypothesis in a study of the femoral head [51, where the loss of trabeculae as reflected by an increase in TbSp, occurred far earlier in the less stressed principal tensile region when compared to the weight bearing principal compressive region. All areas examined within the current study can be considered to have been taken from the ‘prinTable 10 Correlation @V/TV)
between
the percentage
bone volume
fraction
cipal compressive’ regions of the patellofemoral and tibiofemoral joints. This may explain the absence of any significant difference in BV/TV between the sexes, since in the principal compressive region of the femoral head no such difference in BV/TV between males and females existed [5]. BS/TV showed no variation with age, in contrast to previous studies of the iliac crest [81 and the femoral head [3] where a significant decrease with age was seen. In one study of the iliac crest there was a decrease in BS/TV only after the age of 60 [13]. This discrepancy of the effect of age on BS/TV, when compared to other regions of the skeleton, may be related to the fact that few trabeculae are completely lost from the cancellous network during age-related bone loss within the knee. The observation that the BV/TV of the subchondral plate correlated strongly with both the BV/TV of cancellous bone and Tb.Th is similar to the result obtained by Noble and Mexander [14] in the tibia1 plateau. In the present study the correlation was significant only in the medial tibia1 plateau and the medial femoral condyle. Thus the hypothesis of strong pillars being necessary to uphold a thicker subchondral plate [14] appears to be valid only for regions of
of the subchondral
plate
(SCB)
and the bone
volume
Region
n
i-
P
Correlation
M F MF MT TR PA
16 48 31 31 31 31
0.248 0.577 0.671 0.405 - 0.047 0.329
< 0.05 < 0.0001 < 0.0001 < 0.05 NS NS
SCB SCB SCB SCB SCB SCB
M, males;
F, females;
MF.
medial
femur;
MT, medial
tibia;
TR, Trochlea;
PA, Patella.
= = = = = =
fraction
of cancellous
46.2 + 0.5 x BV/TV 22.4 + 1.3 x BV/TV 0.5 + 1.7 x BV/TV 52.5 + 0.6 x BV/TV 60.3 - 0.01 x BV/TV 50.5 + 0.5 x BV/TV
bone
B. Kosiyca Table 11 Correlation
between
the percentage
bone
volume
fraction
et al. / The Knee 3 (1996)
18-22
of the subchondial
(SCB)
plate
21
and trabecular
thickness
Region
n
r
P
Correlation
M F MF MT TR PA
76 48 31 31 31 31
0.223 0.676 0.630 0.369 0.109 0.253
< 0.05 < 0.0001 < 0.0001 < 0.05 NS NS
SCB SCB SCB SCB SCB SCB
M, males;
F, females;
MF,
medial
femur;
MT,
medial
tibia;
TR, Trochlea;
the knee directly involved in weight bearing. The thinning of the subchondral plate as cancellous BV/TV decreases is more rapid in females than in males (P < 0.01); and whilst cancellous BV/TV accounts for only 6% of the variance in subchondral plate BV/TV in males, the figure rises toI 33% in females. This may render women more susceptible to fracture as a result of age-related bone loss. The marked density of the tibia1 subchondral Iplate appears related to its mechanically weak concave surface, where loading acts to separate the constituent elements [15]. Furthermore, as BV/TV decreases in the medial tibia with increasing age, it appears the loss of mineralised bone is mainly from the cancellous bone and not from the subchondral plate where mineralised bone remains to maintain the structural integrity of the concave medial tibia1 plateau [151. A number of hypotheses have been put forward regarding the role of calcified cartilage in synovial joints. It has been suggested that it represents a zone of transitional stiffness, decreasing the stress concentration at the bone-cartilage interface [16,17]. It has been proposed to bond cartilage and bone firmly together by firmly anchoring the collagen fibres of hyaline cartilage [15]. The interdigitation of the calcified layer and the underlying bone has also been proposed to have a role in the conversion of shear stresses to compressive forces [181. The presence of a significant correlation between calcified cartilage thickness and both BV/TV and TbTh may be the result of the fact that as bone becomes more dense, with thicker trabeculae, it will become less compliant [l] and will then require a greater zone of transitional stiffness to minimise cartilage damage. If this is so it is not clear why this relationship is significant only in males but this may be related to the more profound effect of age on bone volume in females, masking any relationship with the calcified layer. Despite not being involved in direct weight bearing the patella has a much denser trabecular network than the other regions of the knee examined. This unusual feature may be the result of the considerable forces to which the patella is exposed, tethered at one end to the tibia and subject to the tensile forfces of the
= = = = = =
(Tb.Th)
in micrometres
47.3 + 0.1 x Tb.Th 16.1 + 0.4 x Tb.Th 4.8 + 0.4 x Tb.Th 50.8 + 0.2 x Tb.Th 53.1 + 0.05 x Tb.Th 54.6 + 0.1 x Tb.Th
PA, Patella.
quadriceps femoris. The dense bone of the patella will render the cartilage susceptible to potentially damaging loading forces [l] and this may explain the extensive cartilage damage typically seen in the patella [19,20]. Although cancellous bone is important in attenuating loading and weight bearing forces, the patella is not directly involved in weight bearing and, therefore, it cannot be assumed that the cancellous bone in the patella has the same importance in absorbing potentially damaging forces. The patella moves across the medial femoral condyle, the trochlea and the subsynovial fat pad [21] and thus would appear to be more exposed to shear forces and the role of cancellous bone in dealing with such forces is not clear. That there is a relationship between bone and cartilage condition in the patella is supported by the work of E&stein [22] on cartilage damage in the patellas of young adults; cartilage damage on the lateral facet of the patella was associated with a high degree of bone mineralisation and was more likely to progress. In contrast lesions on the medial aspect of the patella were associated with medium to low degrees of bone mineralisation and were less likely to progress. The medial lesions were proposed to be the result of shear stresses. The role of shear forces in the development of cartilage damage is also unclear; synovial joints are almost completely frictionless systems [3] but it seems probable that shearing forces may have a role in the progression of such damage. Calcified cartilage has been proposed to play a role in a joint’s ability to deal with shear forces by anchoring the collagen fibres of the hyaline layer and by providing a transitional zone of stiffness [16,17]. However, it has been shown that increased calcified cartilage thickness is associated with increased shear stress levels in the deepest layers of the articular cartilage [23]. If this is correct it would appear that in the patella as bone becomes denser and calcified cartilage becomes thicker, cartilage is rendered more susceptible to damage from both loading and shear forces. This current study has determined the nature of cancellous bone and bone loss in the knee. The exceptional nature of the cancellous bone of the patella
22
B. Kosqca
et al. /The
has also been described and a possible role for the development of age-related cartilage damage forward. The nature of the relationship between cellous bone, calcified cartilage and cartilage has been discussed.
it in put canalso
Acknowledgements The authors’ wish to thank Mrs B Manthey for her technical advice, Mr I Parkinson for his help in the development of the Quantimet program and Mr Tim Rogers for his technical assistance. References [ll
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