Benjamin J. Ewers

Tissue-Engineered Rotator Cuff Tendon Using Porcine Small Intestine Submucosa Histologic and Mechanical Evaluation in Dogs

To determine its efficacy in stimulating the regeneration of a rotator cuff tendon, an implant of 10-ply porcine small intestinal submucosa was used to replace a completely resected infraspinatus tendon in 21 adult mongrel dogs. The contralateral infraspinatus tendon was elevated and then reattached to the greater tubercle with sutures to mimic conventional repair (sham operation). Mechanical evaluations were performed at 0, 3, and 6 months (five specimens at each time period). Histologic comparisons were made at 3 and 6 months (three specimens). At both times, the gross appearance, histologic continuity, and failure mode of the constructs mimicked those of sham-operated and native infraspinatus tendons, thus suggesting host tissue ingrowth and implant remodeling with solid integration of the regenerated tissue to muscular and bony interfaces. Tissue ingrowth occurred without histologic evidence of foreign body or immune-mediated reactions or adhesions to peripheral tissues. Sham operations simulated tendon mobilization and reimplantation procedures routinely performed to treat chronic rotator cuff tendon injuries. Although the ultimate strength of small intestinal submucosa-regenerated tendons was significantly less than that of native infraspinatus tendons (P<0.001), it was similar to that of reimplanted tendons at 3 (P<0.05) and 6 months (P<0.05).

The extent of matrix damage and chondrocyte death in mechanically traumatized articular cartilage explants depends on rate of loading

Mechanical loads can lead to matrix damage and chondrocyte death in articular cartilage. This damage has been implicated in the pathogenesis of secondary osteoarthritis. Studies on cartilage explants with the attachment of underlying bone at high rates of loading have documented cell death adjacent to surface lesions. On the other hand, studies involving explants removed from bone at low rates of loading suggest no clear spatial association between cell death and matrix damage. The current study hypothesized that the observed differences in the distribution of cell death in these studies are attributed to the rate of loading. Ninety bovine cartilage explants were cultured for two days. Sixty explants were loaded in unconfined compression to 40 MPa in either a fast rate of loading experiment (∼900 MPa/s) or a low rate of loading experiment (40 MPa/s). The remaining 30 explants served as a control population. All explants were cultured for four days after loading. Matrix damage was assessed by measuring the total length and average depth of surface lesions and the release of glycosaminoglycans to the culture media. Explants were sectioned and stained with calcein and ethidium bromide homodimer to document the number of live and dead cells. Greater matrix damage was documented in explants subjected to a high rate of loading, compared to explants exposed to a low rate of loading. The high rate of loading experiments resulted in cell death adjacent to fissures, whereas more dead cells were observed in the low rate of loading experiments and a more diffuse distribution of dead cells was observed away from the fissures. In conclusion, this study indicated that the rate of loading can significantly affect the degree of matrix damage, the distribution of dead cells, and the amount of cell death in unconfined compression experiments on explants of articular cartilage.

THE TENSILE AND STRESS RELAXATION RESPONSES OF HUMAN PATELLAR TENDON VARIES WITH SPECIMEN CROSS-SECTIONAL AREA

In order to provide insight into the mechanical response of the collagen fascicle structures in tendon, a series of constant strain rate and constant displacement, stress relaxation mechanical tests were performed on sequentially sectioned human patellar tendon specimens (protocol 1) and specimens with both small (approximately 1 mm2) and large (approximately 20 mm2) cross-sectional areas (protocol 2). These data described the stress relaxation and constant strain rate tensile responses as a function of cross-sectional area and water content. The experimental data suggested that small portions of tendon exhibit a higher tensile modulus, a slower rate of relaxation and a lower amount of relaxation in comparison to larger specimens from the same location in the same tendon. The decrease in relaxation response and the increase in tensile modulus with decreasing cross-sectional area was nonlinear. These data suggest that there may be structures other than the subfascicle, such as the epitenon and other connective tissue components, which influence the tensile and stress relaxation responses in tendon.

Chronic changes in rabbit retro-patellar cartilage and subchondral bone after blunt impact loading of the patellofemoral joint

Animal models of acute joint injury are useful for study of changes in joint tissues that may eventually lead to degradative disease. Our laboratory has developed a joint trauma model using a single blunt impact to the patellofemoral joint of rabbits and documented softening of retro‐patellar cartilage and thickening of its underlying bone out to 12 months post‐trauma. In the present study, we examined changes in these joint tissues out to 36 months post‐impact. Forty‐nine Flemish giant rabbits were impacted on the right patellofemoral joint and sacrificed at one of six times: 0, 4.5, 7.5, 12, 24, and 36 months post‐impact. A 30% reduction in the compressive modulus of traumatized retro‐patellar cartilage occurred at 4.5 months versus the contralateral, non‐impacted limb and remained at this reduced level out to 36 months. The fluid permeability of traumatized cartilage also increased over time from baseline and versus the non‐impacted limb. Tissue thickness increased slightly at 4.5 months and then decreased over time to a 45% difference from baseline at 36 months post‐trauma. While impacted cartilage revealed a significantly greater length of surface fissuring than contralateral, non‐impacted cartilage, no time‐dependent changes were evident in this study. Moreover, the number and depth of these impact surface lesions did not change as a function of time. Finally, the histological analyses indicated that the thickness of underlying subchondral bone increased over time from baseline and versus that in the non‐impacted limb. This long‐term study suggested an association between a decrease in the characteristic time constant of traumatized cartilage and thickening of the underlying subchondral bone. Any potential cause and effect relationship, however, must be investigated in future studies.

The extent and distribution of cell death and matrix damage in impacted chondral explants varies with the presence of underlying bone.

Excessive mechanical loading can lead to matrix damage and chondrocyte death in articular cartilage. Previous studies on chondral and osteochondral explants have not clearly distinguished to what extent the degree and the distribution of cell death are dependent on the presence of an underlying layer of bone. The current study hypothesized that the presence of underlying bone would decrease the amount of matrix damage and cell death. Chondral and osteochondral explants were loaded to 30 MPa at a high rate of loading (approximately 600 MPa/s) or at a low rate of loading (30 MPa/s). After 24 hours in culture, matrix damage was assessed by the total length and average depth of surface fissures. The explants were also sectioned and stained for cell viability in the various layers of the cartilage. More matrix damage was documented in chondral than osteochondral explants for each rate of loading experiment. The total amount of cell death was also less in osteochondral explants than chondral explants. The presence of underlying bone significantly reduced the extent of cell death in all zones in low rate of loading tests. The percentage of cell death was also reduced in the intermediate zone and deep zones of the explant by the presence of the underlying bone for a high rate of loading. This study indicated that the presence of underlying bone significantly limited the degree of matrix damage and cell death, and also affected the distribution of dead cells through the explant thickness. These data may have relevance to the applicability of experimental data from chondral explants to the in situ condition.

Chronic softening of cartilage without thickening of underlying bone in a joint trauma model

We have recently developed a trauma model to study degradation of the rabbit patello-femoral joint. Our current working hypothesis is that alterations in retropatellar cartilage and underlying bone in our model are initiated independently by acute overstresses developed in each tissue during blunt insult to the joint, and that the processes of chronic degradation in each tissue are not related in a mechanical sense. The current study was conducted in an attempt to help validate our hypothesis by impacting the patello-femoral joint with a padded interface. Based upon earlier human cadaver experiments, we believe this would reduce the acute overstresses in patellar bone while the stresses developed in the overlying retropatellar cartilage would be sufficient enough to initiate a chronic softening of the tissue. Twenty-four animals received an impact to the patello-femoral joint and were sacrificed at either 0, 4.5, or 12 months post-insult. Three acute animals were impacted to develop a simplified computational model to estimate the stresses in joint tissues. The study showed there was a significant softening of the retropatellar cartilage at 4.5 and 12 months post-trauma, compared to unimpacted controls. However, no thickening of the underlying subchondral bone was documented at any timepoint. This was consistent with a reduction of stress in the bone compared to earlier studies, which document thickened subchondral bone post-insult at the same applied impact load. In conclusion, this study helped validate our hypothesis by documenting chronic softening of cartilage without remodeling of the underlying subchondral bone. Furthermore, this study, along with our earlier studies, suggest that impact load alone, which is currently used by the automobile industry to certify new automobiles, is not a good predictor of chronic injuries to a diarthrodial joint, and that simply the addition of padding to impact interfaces may not be adequate to protect occupants from chronic injuries.

Impact orientation can significantly affect the outcome of a blunt impact to the rabbit patellofemoral joint.

This laboratory has developed a subfracture, joint trauma model in rabbits. Using a dropped impact mass directed onto a slightly abducted joint, chronic softening of retropatellar cartilage and thickening of underlying subchondral bone are documented in studies to 1 year post-insult. It has been hypothesized that these tissue changes are initiated by stresses developed during impact loading. A previous analytical study by this laboratory suggests that tensile strains in retropatellar cartilage can be significantly lowered, without significantly changing the intensity of stresses in the underlying subchondral bone, by reorientation of patellar impact more centrally on the joint. In the current study comparative experiments were performed on groups of animals after either an impact directed on the slightly abducted limb or a more central impact. One-year post-trauma in animals subjected to the central-oriented impact no degradation of the shear modulus for the retropatellar cartilage was documented, but the thickness of the underlying subchondral bone was significantly increased. In contrast, alterations in cartilage and underlying bone following impact on the slightly abducted limb were consistent with previous studies. The current experimental investigation showed the sensitivity of post-trauma alterations in joint tissues to slight changes in the orientation of impact load on the joint. Interestingly, for this trauma model thickening of the underlying subchondral plate occurred without mechanical degradation of the overlying articular cartilage. This supports the current laboratory hypothesis that alterations in the subchondral bone and overlying cartilage occur independently in this animal model.

Blunt Injuries to the Patellofemoral Joint Resulting From Transarticular Loading Are Influenced by Impactor Energy and Mass

Various impact models have been used to study the injury mechanics of blunt trauma to diarthrodial joints. The current study was designed to study the relationship between impactor energy and mass on impact biomechanics and injury modalities for a specific test condition and protocol. A total of 48 isolated canine knees were impacted once with one of three free flight inertial masses (0.7, 1.5, or 4.8 kg) at one of three energy levels (2, 11, 22 J). Joint impact biomechanics (peak load, loading rate, contact area) generally increased with increasing energy. Injuries were typically more frequent and more severe with the larger mass at each energy level. Histological analyses of the patellae revealed cartilage injuries at low energy with deep injuries in underlying bone at higher energies.

Erratum to: Rate of blunt impact loading affects changes in retropatellar cartilage and underlying bone in the rabbit patella [Journal of Biomechanics 35(6) (2002) 747–755]

Fig. 1. The typical load–time response for the high rate of loading experiments was a nearly symmetrical haversine, while the low rate of loading experiments resulted in a skewed haversine. There were no significant differences in peak load between the high and low rates of loading experiments. However, time to peak and contact duration were significantly greater in the low versus high rate of loading experiments.