THE DISTRIBUTION OF CANCELLOUS BONE WITHIN THE NORMAL AND OSTEOPOROTIC PROXIMAL TIBIA:
IMPLICATIONS FOR FIXATION IN TKR
+*Noble, P; **Conditt, M; *Antonacci, D
+*Baylor College of Medicine, Houston, Texas. 6550 Fannin, Suite 2625\Houston, Texas 77030, 713-986-5462, Fax: 713-797-6658, [email protected]
Materials and Methods: Ten fresh tibiae were obtained from cadaveric
donors ranging from 43 to 93 years of age (5 males, 5 females). Six
specimens were graded as normal, four as osteoporotic. Eight transverse
slices (thickness: 5mm) were cut from each specimen from the medial plateau
to the tibial tubercle. High resolution contact radiographs were prepared of
each slice and digitized. Image analysis software generated colored contour
maps of each slice to determine the average bone density at 50-70,000 points
per slice relative to 12 cancellous and cortical bone standards of known
density. A geometric model of the proximal tibia generated six different
anatomic sections defined by their distinct locations and bone densities
identified as medial, lateral, central, anterior, anterior-lateral, and posterior
(Figure 1). The cortical thickness was also measured along 8 rays separated
by 45° (Figure 2). The distribution of mineralized bone was correlated with
the age and sex of each donor and the degree of osteoporosis.
Results: The average trabecular density of the ten metaphyseal specimens
ranged from 0.91 to 1.51 gm/cm3. The densest bone (2.12 ± 0.13 gm/cm3)
was located posteriorly between the tibial plateaus (zone 6) and within the
anteromedial corner (zone 5) beneath the iliotibial tract (1.84 ± 0.11 gm/cm3).
In the normal tibae, cancellous bone within the medial plateau (zone 1) had an
average density of 1.35 ± 0.06 gm/cm3, 16% higher than the lateral plateau
(zone 2, 1.16 ± 0.09 gm/cm3), while in the osteoporotic specimens, the medial
and lateral plateaus were of identical bone density (Figure 3). In both normal
and osteoporotic specimens, the weakest bone was found within the central
third (zone 3, 0.7 ± 0.03 gm/cm3) and the anterior third (zone 4, 0.96 ± 0.06
gm/cm3) of the tibial metaphysis. With increasing severity of osteoporosis, a
large central/anterior zone of weak cancellous bone developed with loss of the
normal gradient of cancellous density from proximal to distal. Finally,
cortical thickness was significantly higher in the posterior section of the tibial
plateau (Figure 4).
Cancellous Density (gm/cm3)
Introduction: Large-scale studies of the outcome and survival of total knee
replacements indicate that aseptic loosening of cemented tibial components is
more common than has been previously assumed. Probable causes of
loosening include variations in cementing technique, implant design and
exposure of bone of adequate strength within the proximal tibial metaphysis.
This becomes especially critical in the osteoporotic tibia. In this study, we
quantified the three-dimensional distribution of cortical and cancellous bone
within the proximal tibia as a function of age, sex and the severity of
osteoporosis.
1.4
Normal
Osteoporotic
1.2
1.0
0.8
0.6
0.4
M edial
Plateau
Lateral
Plateau
Central
Zone
Anterior
Zone
Figure 3. Cancellous Density: Effect of Osteoporosis
Figure 1. Slice Divided Into Six Zones of Similar Density
1
Normal
Osteoporotic
4.0
3.0
Level of Section: 10 mm
2.0
1.0
0.0
2
8
Cortical Thickness (mm)
5.0
Anterior
Lateral
Posterior
M edial
Figure 4. Cortical Thickness: Effect of Osteoporosis
7
3
6
5
4
Figure 2. Measurement of Cortical Thickness
Discussion: A peripheral ring of cancellous bone exists in the proximal tibia
of much greater mechanical consequence than the relatively thin shell of
cortical bone. This suggests that "cortical overlap" is less important than
originally thought for direct support of prosthetic components. However, with
increasing bone loss, cancellous bone capable of supporting prosthetic devices
becomes progressively more peripheral and is limited to the proximal 10mm
of the tibia. Our results suggest that, in the osteoporotic tibia, improved
cement fixation could be accomplished by resecting the tibia at a level as
proximal as possible. In addition, manual removal of all of the weak
cancellous bone from the anterior and central zones is recommended,
regardless of the shape and size of the keel beneath the tibial tray.
**Institute of Orthopedic Research and Education, Houston, Texas.
Poster Session - Knee Arthroplasty - VALENCIA FOYER
0456
46th Annual Meeting, Orthopaedic Research Society, March 12-15, 2000, Orlando, Florida