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Research Papers

# Chondrocyte Damage and Contact Pressures Following Impact on the Rabbit Tibiofemoral Joint

[+] Author and Article Information
Daniel I. Isaac, Eric G. Meyer

Orthopaedic Biomechanics Laboratories, College of Osteopathic Medicine, Michigan State University, East Lansing, MI 48824

Roger C. Haut

Orthopaedic Biomechanics Laboratories, College of Osteopathic Medicine, Michigan State University, East Lansing, MI 48824haut@msu.edu

J Biomech Eng 130(4), 041018 (Jun 23, 2008) (5 pages) doi:10.1115/1.2948403 History: Received July 16, 2007; Revised May 09, 2008; Published June 23, 2008

## Abstract

Epidemiological studies show that tibial plateau fractures comprise about 10% of all below-knee injuries in car crashes. Studies from this laboratory document that impacts to the tibiofemoral (TF) joint at 50% of the energy producing gross fracture can generate cartilage damage and microcracks at the interface between calcified cartilage and underlying subchondral bone in the tibial plateau. These injuries are suggestive of the initiation for a long term chronic disease, such as osteoarthritis. The disease process may be further encouraged by acute damage to chondrocytes in the cartilage overlying areas of occult microcracking. The hypothesis of the current study was that significant damage to chondrocytes in tibial plateau cartilage could be generated in areas of high contact pressure by a single impact delivered to the rabbit TF joint, without a gross fracture of bone. Three rabbits received a single, $13J$ of energy blunt insult to the TF joint, while another three animals were used as controls. Cell viability analyses compared chondrocyte damage in impacted versus control cartilage. Two additional rabbits were impacted to document contact pressures generated in the TF joint. The study showed high contact pressures in uncovered areas of the plateau, with a trend for higher pressures in the lateral versus medial facets. A significantly higher percentage of damaged chondrocytes existed in impacted versus the opposite, nonimpacted limbs. Additionally, more chondrocyte damage was documented in the superficial zone (top 20% of cartilage thickness) of the cartilage compared to middle (middle 50% of thickness) and deep (bottom 30% of thickness) zones. This study showed that a single blunt insult to the in situ rabbit TF joint, generating large areas of contact pressure exceeding $20MPa$, produces significant chondrocyte damage in the tibial articular cartilage, especially in the superficial zone, without gross fracture of bone. Future studies will be needed to investigate the long term, chronic outcome of this blunt force joint trauma.

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## Figures

Figure 1

The drop tower fixture consisted of a slide track designed to prevent rotation of the dropped sled during impact. After a single impact, the sled was arrested electronically by an electromagnetic catching device. The impact interface was a precrushed, deformable surface (Hexcel, 3.76MPa crush strength) mounted in front of a 4.45 kN (1000lb) load transducer.

Figure 2

Impact experiments were performed by dropping a gravity-accelerated mass onto the flexed tibial-femoral joint with approximately 13J of potential energy. The rabbit was oriented such that the deformable interface struck the distal femur with impact forces oriented axially in the tibia.

Figure 3

The posterior half of the subchondral bone was glued to a rectangular aluminum block, which was attached to a rotary microtome. Approximately 7–10min of drying time was allowed, as PBS was continually applied to the cartilage surface. Approximately 18 slices, each 150μm thick, were taken from each facet for analysis.

Figure 4

Impact induced contact pressure distributions and contact areas in the tibial femoral joint were measured by pressure sensitive film. Mapping the pressure distributions onto the tibial plateau showed that the location of highest contact pressures was largely in the area not covered by the meniscus.

Figure 5

The percentage of cells with damaged membranes was manually quantified using an image processing and analysis program. Significantly more damaged cells were observed in both the medial and lateral facets of the impacted samples when compared to the opposite, nonimpacted limbs. Statistical differences in the percentage of cells with damaged membranes are denoted by an asterisk. Statistical differences were found using two-factor repeated measures ANOVA with p<0.05 for statistical significance.

Figure 6

The stained osteochondral explants were imaged and divided into three zones: superficial, middle, and deep. Cell viability was measured in the thin sections of cartilage and bone.

Figure 7

Significantly more damaged cells were observed in the superficial layer of the lateral facet when compared to the middle and deep zones. Also, significantly more damaged cells were observed in the superficial zone of the medial facet when compared to the deep zone; however, no difference was observed when the superficial zone was compared to the middle zone. Statistical differences in the percentage of dead cells were denoted by an asterisk. The statistical analyses were based on two-factor ANOVA’s with p<0.05 for statistical significance.

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