A study of acute and chronic tissue changes in surgical and traumatically-induced experimental models of knee joint injury using magnetic resonance imaging and micro-computed tomography.

OBJECTIVE: The objective of this study was to monitor the progression of joint damage in two animal models of knee joint trauma using two non-invasive, clinically available imaging modalities. METHODS: A 3-T clinical magnet and micro-computed tomography (µCT) was used to document changes immediately following injury (acute) and post-injury (chronic) at time points of 4, 8, or 12 weeks. Joint damage was recorded at dissection and compared to the chronic magnetic resonance imaging (MRI) record. Fifteen Flemish Giant rabbits were subjected to a single tibiofemoral compressive impact (ACLF), and 18 underwent a combination of anterior cruciate ligament (ACL) and meniscal transection (mACLT). RESULTS: All ACLF animals experienced ACL rupture, and 13 also experienced acute meniscal damage. All ACLF and mACLT animals showed meniscal and articular cartilage damages at dissection. Meniscal damage was documented as early as 4 weeks and worsened in 87% of the ACLF animals and 71% of the mACLT animals. Acute cartilage damage also developed further and increased in occurrence with time in both models. A progressive decrease in bone quantity and quality was documented in both models. The MRI data closely aligned with dissection notes suggesting this clinical tool may be a non-invasive method for documenting joint damage in lapine models of knee joint trauma. CONCLUSIONS: The study investigates the acute to chronic progression of meniscal and cartilage damage at various time points, and chronic changes to the underlying bone in two models of posttraumatic osteoarthritis (PTOA), and highlights the dependency of the model on the location, type, and progression of damage over time.

Housing conditions alter properties of the tibia and humerus during the laying phase in Lohmann white Leghorn hens.

Osteoporosis in caged hens is one driving factor for the United States egg industry to explore options regarding alternative housing systems for laying hens. The aim of our research was to study the influence of housing systems on tibiae and humeri of 77-week-old Lohmann White hens. Pullets raised in an aviary system were either continued in aviary hen systems (AV) or conventional cages (AC) whereas pullets reared in conventional cages continued in conventional hen cages (CC) or enriched colony cages (EN) at 19 weeks. From each group, 120 hens were randomly euthanized and right and left tibae and humeri were excised for structural and mechanical analysis. Volumetric density of the cortical bone was measured using quantitative computed tomography (QCT). Aviary (AV) hens had greater cortical thickness and density but similar outer dimensions to AC hens (P < 0.05). Hens in EN system had humeri with similar cortical thickness and density but wider outer dimensions than the humeri of CC hens (P < 0.05). Cortical geometry of the tibiae was the same for the EN and CC hens, whereas EN hens had denser tibial cortex than CC hens (P < 0.05). Geometrical changes in the humeri suggest that hens in the AV system were better able to protect their structure from endosteal resorption during the laying phase. Humeri of AV and EN hens had increased second moment of area compared to the AC and CC hens; however, the changes were not observed in tibiae. Mechanical property differences were observed, with bones of AV hens having greater failure moment and stiffness than AC hens and the same difference was observed between the EN and CC hens, (P < 0.05). These findings indicate that movement limitation causes loss of bone mass and density whereas provision of moderate movement increases certain bone quality parameters during adulthood in laying hens.

Effect of rearing environment on bone growth of pullets.

Alternative housing systems for laying hens provide mechanical loading and help reduce bone loss. Moreover, achieving greater peak bone mass during pullet phase can be crucial to prevent fractures in the production period. The aim of this study was to determine the housing system effects on bone quality of pullets. Tibiae and humeri of White Leghorn pullets reared in conventional cages (CCs) and a cage-free aviary (AV) system were studied. At 16 wk, 120 birds at random from each housing system were euthanized. Right and left tibiae and humeri were collected and further analyzed. Cortical bone density and thickness were measured using computed tomography. Periosteal and endosteal dimensions were measured at the fracture site during mechanical testing. At 4, 8, 12, and 16 wk, serum concentrations of osteocalcin and hydroxylysyl pyridinoline were analyzed as markers of bone formation and resorption. Cortical bone density was higher (P<0.05) in humeri of AV pullets, and tibiae were denser (P<0.05) for AV pullets in the distal section of the bone compared to CC pullets. Ash content was higher (P<0.05) in AV humeri with no difference in tibiae ash content. Tibiae and humeri of AV pullets had a thicker cortex than the CC pullets (P<0.05). Additionally, the tibiae and humeri of AV pullets had greater (P<0.05) second moment of areas than the CC pullets. While some bone material properties between groups were different (P<0.05), the differences were so small (<7%) that they likely have no clinical significance. Serum osteocalcin concentrations were not different between the treatments, but hydroxylsyl pyridinoline concentrations were higher in CC pullets at 12 wk compared to the AV pullets and the effect reversed at 16 wk (P<0.05). These findings indicate that tibiae and humeri respond differently to load bearing activities during growth. The improved load bearing capability and stiffness in bones of AV pullets were related to increased cross-sectional geometry.

Bone characteristics and femoral strength in commercial toms: the effect of protein and energy restriction.

Selection for rapid growth in turkeys has resulted in skeletal problems such as femoral fractures. Slowing growth rate has improved bone structure, but the effect on mechanical properties of the bone is unclear. The current study's hypothesis was that slowing the growth of turkeys by reducing energy and CP in the diet would result in increased femur integrity. Commercial turkeys were fed 1 of 3 diets: control with 100% of NRC energy and CP levels, as well as a diet feeding 80 or 60% of NRC energy and CP levels. All other nutrients met or exceeded NRC requirements. Control birds were grown to 20 wk of age, whereas the 80 and 60% NRC birds were sampled when BW matched that of control birds at wk 4, 8, 12, 16, and 20. Both femurs were extracted, with one being measured and ashed and the other twisted to failure to evaluate mechanical properties. Total bone length, diameter, cortical thickness, and cortical density were measured. The total femur length was longer in the 60% NRC birds at 5 and 10 kg of BW compared with control (P < 0.05); this significance was lost by the time birds reached 16 kg of BW. At 5 and 10 kg of BW, ash content was higher in the control birds than in the 60% NRC birds (P < 0.05). At 16 kg of BW, the 60% NRC birds had the highest femur ash (P < 0.05). The mechanical testing parameters were failure torque, shear strength, and shear modulus of the bones. The 60% diet produced the highest failure torque (P < 0.05), at 16 kg of BW and onward. The shear strength was greater (P = 0.01) once the birds reached 5 kg of BW for the 60% diet than other diets. In conclusion, reducing the energy and protein in the diet to 60% of NRC recommendations, thus slowing growth, improved bone strength, as measured by failure torque, and bone quality, as measured by shear strength, without altering bone length or ash content by the time birds reached market weight.

A Morphological Study of the Meniscus, Cartilage and Subchondral Bone Following Closed-Joint Traumatic Impact to the Knee.

Post-traumatic osteoarthritis (PTOA) is a debilitating disease that is a result of a breakdown of knee joint tissues following traumatic impact. The interplay of how these tissues influence each other has received little attention because of complex interactions. This study was designed to correlate the degeneration of the menisci, cartilage and subchondral bone following an acute traumatic event that resulted in anterior cruciate ligament (ACL) and medial meniscus tears. We used a well-defined impact injury animal model that ruptures the ACL and tears the menisci. Subsequently, the knee joints underwent ACL reconstruction and morphological analyses were performed on the menisci, cartilage and subchondral bone at 1-, 3- and 6-months following injury. The results showed that the morphological scores of the medial and lateral menisci worsened with time, as did the tibial plateau and femoral condyle articular cartilage scores. The medial meniscus was significantly correlated to the medial tibial subchondral bone at 1 month (p = 0.01), and to the medial tibial cartilage at 3 months (p = 0.04). There was only one significant correlation in the lateral hemijoint, i.e., the lateral tibial cartilage to the lateral tibial subchondral bone at 6 months (p = 0.05). These data may suggest that, following trauma, the observed medial meniscal damage should be treated acutely by means other than a full or partial meniscectomy, since that procedure may have been the primary cause of degenerative changes in the underlying cartilage and subchondral bone. In addition to potentially treating meniscal damage differently, improvements could be made in optimizing treatment of acute knee trauma.

Effect of bending direction on the mechanical behaviour of interlocking nail systems.

OBJECTIVES: To compare the mechanical properties of various interlocking nail constructs in medio-lateral (ML) and cranio-caudal (CC) bending. METHODS: Synthetic bone models simulating a severely comminuted tibial fracture were treated with either screwed or bolted, 6 or 8 mm standard interlocking nails (ILN), or an angle-stable ILN (AS-ILN), after which they were then sequentially tested in ML and CC bending. Construct compliance, maximum angular deformation (MaxDef) and slack were statistically compared (p<0.05). RESULTS: The compliance of all constructs was significantly greater in CC than in ML bending. However, due to the presence of a greater slack in the ML plane, standard ILN constructs sustained significantly more deformation in that plane. Maximum deformation of the novel AS-ILN constructs was the smallest of all constructs and consistently occurred without slack regardless of bending direction. CLINICAL SIGNIFICANCE: This study suggested that standard ILN construct overall deformation and acute instability (slack) may be more critical in ML than in CC bending. Conversely, the small MaxDef and the absence of slack in both bending planes seen in novel angle-stable AS-ILN may provide optimal construct stability and in turn may be more conducive to bone healing.

Association of knee bone bruise frequency with time postinjury and type of soft tissue injury.

This study correlated the frequency of bone bruises with soft tissue injuries in the knee and examined bruise frequency as a function of time postinjury. Magnetic resonance imaging of 1546 patients revealed bone bruises in 19% of patients without an associated meniscal or ligamentous injury. For those patients presenting with at least one meniscoligamentous injury, the frequency of bruising was 60% at 0 to 4 weeks, 42% at 4 to 10 weeks, and 31% at 10 to 26 weeks postinjury. The frequency of bruising varied with the presence of concomitant injuries, with the greatest frequency of bruises (78%) observed in patients with anterior cruciate ligament injuries.

Influence of age and housing systems on properties of tibia and humerus of Lohmann White hens1: Bone properties of laying hens in commercial housing systems.

This study was aimed at analyzing bone properties of Lohmann White hens in different commercial housing systems at various points throughout production. Pullets reared in conventional cages (CC) were either continued in CC or moved to enriched colony cages (EN) at 19 weeks. Pullets reared in cage-free aviaries (AV) were moved to AV hen houses. Bone samples were collected from 60 hens at each of 18 and 72 wk and 30 hens at 26 and 56 wk from each housing system. Left tibiae and humeri were broken under uniform bending to analyze mechanical properties. Cortical geometry was analyzed using digital calipers at the fracture site. Contralateral tibiae and humeri were used for measurement of ash percentage. AV pullets' humeri had 41% greater cortical areas, and tibiae had 19% greater cortical areas than the CC pullets (P < 0.05). Average humeri diameter was greater in AV pullets than in CC pullets (P < 0.05), whereas the tibiae outer dimensions were similar. Aviary pullet bones had greater stiffness (31 and 7% greater for tibiae and humeri, respectively) and second moment of inertia (43 and 13% greater for tibiae and humeri, respectively) than CC pullets (P < 0.05). The differences between bones of AV and CC hens persisted throughout the laying cycle. Moving CC pullets to EN resulted in decreased endosteal resorption in humeri, evident by a 7.5% greater cortical area in the EN hens (P < 0.05). Whole-bone breaking strength did not change with age. Stiffness increased with age, while energy to failure decreased in both the tibiae and humeri. These results indicated that tibiae and humeri of laying hens become stiffer but lose toughness and become brittle with age. Furthermore, AV and EN systems can bring positive changes in mechanical and structural properties that are more pronounced in the humerus than the tibia.

Exposure to a standard culture medium alters the response of cartilage explants to injurious unconfined compression.

Previous studies on chondral explants have not clearly described to what extent the degree and the distribution of cell death are dependent on the amount of free swelling seen during tissue equilibration in a standard culture medium. The current study hypothesized that increased fluid content inside equilibrated chondral explants, when subjected to injurious compression, would lead to greater matrix damage during unconfined compression. Equilibrated and non-equilibrated chondral explants were loaded to 30 MPa at a fast rate of loading ( approximately 600 MPa/s). Stress-strain curves were documented for each explant. Matrix damage was assessed by the length of surface fissures. Chondrocyte viability was also measured in the various layers of the explants. The stiffness of the equilibrated specimens was less than non-equilibrated specimens, and it correlated with the amount of fluid absorbed during equilibration. More matrix damage and associated cell death in the superficial zone were documented in equilibrated than non-equilibrated explants, and these correlated positively with fluid absorbed during equilibration. This study indicated that equilibration of chondral explants in a standard culture medium alters their response to mechanical loading in terms of stiffness, matrix damage and cell viability.

Treatment with the non-ionic surfactant poloxamer P188 reduces DNA fragmentation in cells from bovine chondral explants exposed to injurious unconfined compression.

Excessive mechanical loading to a joint has been linked with the development of post-traumatic osteoarthritis (OA). Among the suspected links between impact trauma to a joint and associated degeneration of articular cartilage is an acute reduction in chondrocyte viability. Recently, the non-ionic surfactant poloxamer 188 (P188) has been shown to reduce by approximately 50% the percentage of non-viable chondrocytes 24 h post-injury in chondral explants exposed to 25 MPa of unconfined compression. There is a question whether these acutely 'saved' chondrocytes will continue to degrade over time, as P188 is only thought to act by acute repair of damaged cell membranes. In order to investigate the degradation of traumatized chondrocytes in the longer term, the current study utilized TUNEL staining to document the percentage of cells suffering DNA fragmentation with and without an immediate 24 h period of exposure of the explants to P188 surfactant. In the current study, as in the previous study by this laboratory, chondral explants were excised from bovine metacarpophalangeal joints and subjected to 25 MPa of unconfined compression. TUNEL staining was performed at 1 h, 4 days, and 7 days post-impact. The current study found that P188 was effective in reducing the percentage of cells with DNA fragmentation in impacted explants by approximately 45% at 4 and 7 days post-impact. These data suggest that early P188 intervention was effective in preventing DNA fragmentation of injured chondrocytes. The current hypothesis is that this process was mitigated by the acute repair of damaged plasma membranes by the non-ionic surfactant P188, and that most repaired cells did not continue to degrade as measured by the fragmentation of their DNA.

The limitation of acute necrosis in retro-patellar cartilage after a severe blunt impact to the in vivo rabbit patello-femoral joint.

We have previously shown that surface lesions and acute necrosis of chondrocytes are produced by severe levels of blunt mechanical load, generating contact pressures greater than 25 MPa, on chondral and osteochondral explants. We have also found surface lesions and chronic degradation of retro-patellar cartilage within 3 years following a 6J impact intensity with an associated average pressure of 25 MPa in the rabbit patello-femoral joint. We now hypothesized that cellular necrosis is produced acutely in the retro-patellar cartilage in this model as a result of a 6J impact and that an early injection of the non-ionic surfactant, poloxamer 188 (P188), would significantly reduce the percentage of necrotic cells in the traumatized cartilage. Eighteen rabbits were equally divided into a 'time zero' group and two other groups carried out for 4 days. One '4 day' group was administered a 1.5 ml injection of P188 into the impacted joint immediately after trauma, while the other was injected with a placebo solution. Impact trauma produced surface lesions on retro-patellar cartilage in all groups. Approximately 15% of retro-patellar chondrocytes suffered acute necrosis in the 'time zero' and '4-day no poloxamer' groups. In contrast, significantly fewer cells (7%) suffered necrosis in the poloxamer group, most markedly in the superficial cartilage layer. The use of P188 surfactant early after severe trauma to articular cartilage may allow sufficient time for damaged cells to heal, which may in turn mitigate the potential for post-traumatic osteoarthritis. Additional studies are needed to improve the efficacy of this surfactant and to determine the long-term health of joint cartilage after P188 intervention.

Gene expression profile of mechanically impacted bovine articular cartilage explants.

Traumatic injury to a joint can initiate cartilage degradation. Blunt trauma increases matrix damage and decreases proteoglycan synthesis in in vitro models. Few studies have investigated gene expression of articular cartilage (AC) following mechanical loading. Recent advances in microarray technology allow analysis of a number of genes, and may elucidate pathways of AC degradation. In the present study, we used a bovine cDNA microarray to determine how acute trauma of cartilage explants in the absence of underlying bone alters gene expression. Results indicate that at least 19 genes were differentially expressed at 3 h after trauma. Fourteen genes were up-regulated and five genes were down-regulated relative to control explants. The up-regulated genes included cytokine and chemokine receptors, enzymes, and molecules involved in signal transduction. Genes of adhesion molecules and apoptosis were down-regulated. The results of this study highlight the potential benefits of using a bovine cDNA microarray to study cartilage metabolism.

Effect of bending direction on the mechanical behaviour of 3.5 mm String-of-Pearls and Limited Contact Dynamic Compression Plate constructs.

OBJECTIVE: To compare the bending properties of String-of-Pearls® (SOP) and Limited Contact Dynamic Compression Plate® (LC-DCP) constructs in orthogonal bending directions. METHODS: 3.5 mm SOP and LC-DCP plates were fixed to a bone model simulating a comminuted tibial fracture. Specimens were non-destructively tested in both mediolateral and craniocaudal bending for 10 cycles. Bending stiffness and total angular deformation were compared using parametric analyses (p <0.05). RESULTS: For both constructs, stiffness was significantly less when bending moments were applied against the thickness of the plates (mediolateral bending) than against the width (craniocaudal bending). When compared to the mediolateral plane, bending constructs in the craniocaudal plane resulted in a 49% (SOP group) and 370% (LC-DCP group) increase in stiffness (p <0.001). Mediolateral bending stiffness was significantly greater in the SOP than the LC-DCP constructs. Conversely, in craniocaudal bending, SOP constructs stiffness was significantly less than that of the LC-DCP constructs. The differences between the two constructs in total angular deformation had an identical pattern of significance. CLINICAL SIGNIFICANCE: This study found that SOP showed less variability between the orthogonal bending directions than LC-DCP in a comminuted fracture model, and also described the bi-planar bending behaviour of both constructs. Although not exhibiting identical bending properties in both planes, SOP constructs had a more homogenous bending behaviour in orthogonal loading directions. The difference between the SOP with a circular cross sectional shape compared to the rectangular shape of standard plates is probably responsible for this difference.

Contact pressures in the patellofemoral joint during impact loading on the human flexed knee.

Currently, a bone fracture criterion is used by the automotive industry to assess the potential for a lower extremity injury from impact directed on the flexed knee. However, recent studies with animal models indicate irreversible damage to articular cartilage due to overpressures generated within the patellofemoral (P-F) joint without bone fracture, and suggest this injury may lead to a progressive, degenerative disease of the joint. The purpose of this investigation was to measure contact pressure in the human P-F joint during impact loading on the isolated, flexed knee. Loads were delivered on the patella with a free-flight inertial mass that had a rigid or foam padded interface. The experiments were conducted by serially increasing the impactor velocity in repeated tests until bone fracture was observed. The distribution of maximum pressures generated within the joint was recorded with a pressure-sensitive film. Fracture of the patella or the femur occurred at impact loads of approximately 8.5 kN. The average P-F pressure was approximately 25 MPa for 8 kN of impact load on the 90 degrees flexed joint. The P-F contact area varied with the level of contact load and degree of joint flexion. The distribution of P-F pressures was nonuniform. At approximately 70% of the fracture load for the 90 degrees flexed knee, nearly 35% of the contact area was exposed to pressures greater than 25 MPa. In an earlier study by others using subchondral bone-cartilage preparations, this level of pressure resulted in fissures and lacerations of the cartilage.

Microstructurally based model analysis of gamma-irradiated tendon allografts.

A mathematical model of the collagenous microstructure of tendon was used to analyze the mechanical responses of gamma-irradiated patellar tendon allografts and nonirradiated controls. The model was fit to tensile test response curves from 10 pairs of allografts. Donors ranged in age from 16-64 years (six males, four females). The model indicated a decrease in elastic modulus of irradiated tendon collagen. It also suggested a significant increase in the degree and range of collagen fiber crimping following gamma irradiation. The model results agree with the literature and histological observations. The model fit the experimental response curves well and provided a structurally based, objective method of quantitatively studying the mechanical response of tendons and the consequences of irradiation sterilization.

The effects of test environment and cyclic stretching on the failure properties of human patellar tendons.

There is a need to document the mechanical properties of patellar tendon allografts used for reconstructive surgery of the damaged anterior cruciate ligament, especially the effects of irradiation sterilization. The purpose of this study was to investigate the influences of in vitro test environment and low-level cyclic stretching prior to failure tests on nonirradiated and irradiated human graft tissues. Bilateral patellar tendons were split and each half processed accordingly. Some graft tissues were stretched cyclically at 2.5 mm deformation before failure. Experiments were performed in a 37 degrees C saline bath or with tissues moistened with a drip of the same. The irradiated grafts relaxed less and generated less slack length in the drip environment than the nonirradiated controls. Cyclic stretching did not alter failure characteristics of either graft tissue. While no significant differences in the tensile responses or failure characteristics were noted for irradiated and nonirradiated grafts in the drip, in the bath environment the nonirradiated tissues had greater strength and modulus. This resulted in there being a significant difference between irradiated and nonirradiated tissue responses in a heated saline bath environment. These experimental results exemplify the need to control in vitro test environments in the evaluation of various sterilization and preservation protocols for soft tissue allografts.

Polysulphated glycosaminoglycan treatments can mitigate decreases in stiffness of articular cartilage in a traumatized animal joint.

A single, blunt impact to the rabbit patellofemoral joint has been shown to decrease the stiffness of retropatellar cartilage and increase the thickness of the underlying bone. Polysulphated glycosaminoglycan treatments, on the other hand, have been shown to inhibit the degradation of articular cartilage and possibly increase synthesis of collagen and glycosaminoglycans in experimental studies on diseased joints. The aim of the current study was to examine the effect of early treatments with polysulphated glycosaminoglycans on cartilage using an in vivo post-trauma animal model. The study used 24 Flemish Giant rabbits in three groups: control, impacted, and impacted with treatment. Treatment consisted of intramuscular injections the day of insult and every 4 days thereafter for 6 weeks. At 30 weeks after trauma, mechanical tests were performed on the retropatellar cartilage to determine its mechanical stiffness. The patellae were also grossly evaluated for surface lesions on the retropatellar cartilage and histologically processed to measure the thickness of the subchondral bone. The rabbits that received no treatment had a statistically significant decrease in stiffness (modulus) for the cartilage of the impacted patellae compared with that of the contralateral, unimpacted patellae and compared with the cartilage of rabbits in the control group. The degradation in mechanical stiffness, however, was not observed in patellae of rabbits in the group receiving treatment. There was also a significant increase in the underlying thickness of the subchondral plate on the impacted patellae compared with that on the contralateral, unimpacted sides for rabbits in both the treated and nontreated groups. In conclusion, the polysulphated glycosaminoglycan treatments minimized a decrease in mechanical stiffness (modulus) of retropatellar articular cartilage 30 weeks after trauma. The mechanism by which the mechanical stiffness of the cartilage was preserved is unknown.

Subfracture insult to the human cadaver patellofemoral joint produces occult injury.

The current criterion used by the automotive industry for injury to the lower extremity is based on visible bone fracture. Studies suggest, however, that chronic joint degeneration may occur after subfracture impact loads on the knee. We hypothesized that subfracture loading of the patellofemoral joint could result in previously undocumented microtrauma in areas of high contact pressure. In the current study, seven patellofemoral joints from human cadavers were subjected to impact with successively greater energy until visible fracture was noted. Transverse and comminuted fractures of the patella were noted at 6.7 kN of load. Approximately 45% of the impact energy then was delivered to the contralateral joint. Subfracture loads of 5.2 kN resulted in no gross bone fracture in five of seven specimens. Histological examination of the patellae horizontal split fracture in the subchondral bone, at the tidemark, or at the interface of calcified cartilage and subchondral bone. The trauma appeared predominantly on the lateral facet, adjacent to or directly beneath preexisting fibrillation of the articular surface. Surface fibrillation was noted in histological sections of control patellae (not subjected to impact loading), but occult damages were not observed. Although the mechanism of this occult trauma is unknown, similar damage has been shown to occur from direct shear loading. As these microcracks can potentiate a disease process in the joint, this study may suggest that the current criterion for injury, based on bone fracture alone, is not sufficiently conservative.

Injuries produced by blunt trauma to the human patellofemoral joint vary with flexion angle of the knee.

Patellofemoral joint impact trauma during car accidents, sporting activities, and falls can produce acute gross fracture of bone, microfracture of bone, and soft tissue injury. Field studies of car accidents, however, show that most patellofemoral traumas are classified as 'subfracture' level injuries. While experimental studies have shown that the influence of flexion angle at impact is not well understood, flexion angle may influence injury location and severity. In the current study, 18 pairs of isolated human cadaver knees were subjected to blunt impact at flexion angles of 60 degrees, 90 degrees, or 120 degrees. One knee from each cadaver was sequentially impacted until gross fracture of bone was produced. The contralateral knee was subjected to a single, subfracture impact at 45% of the impact energy producing fracture in the first knee. The fracture experiments produced gross fracture of the patella and femoral condyles with the fracture plane positioned largely within the region of patellofemoral contact. The fracture location and character changed with flexion angle: at higher flexion angles the proximal pole of the patella and the femoral condyles were more susceptible to injury. For the 90 degrees flexion angle, the patella was fractured centrally, while at 60 degrees the distal pole fractured transversely at the insertion of the patellar tendon. In addition, the load magnitude required to produce fracture increased with flexion angle. In the 'subfracture' knees, injuries were documented for all flexion angles; occult microfractures of the subchondral and trabecular bone and fissures of the articular surface. Similar to the fracture-level experiments, the injuries coincided with the patellofemoral contact region. These data show that knee flexion angle plays an important role in impact related knee trauma. Such data may be useful in the clinical setting, as well as in the design of injury prevention strategies.

Blunt impact causes changes in bone and cartilage in a regularly exercised animal model.

The goal of this study was to document the effect of blunt-impact trauma on the knee in a small animal model that incorporated a known level of physical exercise after impact. We hypothesized that a single blunt impact to the patellofemoral joint, of a magnitude comparable with our earlier studies, would result in degenerative changes to cartilage and to subchondral bone of the patella. Blunt impacts were delivered to rabbit patellofemoral joints without producing bone fracture, and biomechanical and histological analyses were performed on joint tissues at various times. At 12 months, the subchondral bone plate was thicker on the impacted side than on the unimpacted side and than that of the controls at comparable locations (near where surface fissures were found on the impacted side). The instantaneous modulus of cartilage was significantly less on the impacted side than that of controls at 3, 6, and 12 months after impact. The relaxed modulus of cartilage on the unimpacted side increased with time after impact and was significantly greater than that of controls at 12 months. These facts suggest that in this exercise model, the contralateral limb should not be considered a control. The retropatellar cartilage on the impacted side was significantly less thick than that of controls at 12 months, and histological analyses of the cartilage and bone indicated early-stage osteoarthrosis in the impacted joint. Thus, in this animal model a single subfracture blunt impact produced degeneration of joint tissues.