Radiographic changes in the knee after meniscal transplantation. An experimental study in a sheep model

Developmental Transformation and Reduction of Connective Cavities within the Subchondral Bone

It is widely accepted that the subchondral bone (SCB) plays a crucial role in the physiopathology of osteoarthritis (OA), although its contribution is still debated. Much of the pre-clinical research on the role of SCB is concentrated on comparative evaluations of healthy vs. early OA or early OA vs. advanced OA cases, while neglecting how pure maturation could change the SCB’s microstructure. To assess the transformations of the healthy SCB from young age to early adulthood, we examined the microstructure and material composition of the medial condyle of the femur in calves (three months) and cattle (18 months) for the calcified cartilage (CC) and the subchondral bone plate (SCBP). The entire subchondral zone (SCZ) was significantly thicker in cattle compared to calves, although the proportion of the CC and SCBP thicknesses were relatively constant. The trabecular number (Tb.N.) and the connectivity density (Conn.D) were significantly higher in the deeper region of the SCZ, while the bone volume fraction (BV/TV), and the degree of anisotropy (DA) were more affected by age rather than the region. The mineralization increased within the first 250 µm of the SCZ irrespective of sample type, and became stable thereafter. Cattle exhibited higher mineralization than calves at all depths, with a mean Ca/P ratio of 1.59 and 1.64 for calves and cattle, respectively. Collectively, these results indicate that the SCZ is highly dynamic at early age, and CC is the most dynamic layer of the SCZ.

Methodology to Produce Specimen-Specific Models of Vertebrae: Application to Different Species

Image-based continuum-level finite element models have been used for bones to evaluate fracture risk and the biomechanical effects of diseases and therapies, capturing both the geometry and tissue mechanical properties. Although models of vertebrae of various species have been developed, an inter-species comparison has not yet been investigated. The purpose of this study was to derive species-specific modelling methods and compare the accuracy of image-based finite element models of vertebrae across species. Vertebral specimens were harvested from porcine (N = 12), ovine (N = 13) and bovine (N = 14) spines. The specimens were experimentally loaded to failure and apparent stiffness values were derived. Image-based finite element models were generated reproducing the experimental protocol. A linear relationship between the element grayscale and elastic modulus was calibrated for each species matching in vitro and in silico stiffness values, and validated on independent sets of models. The accuracy of these relationships were compared across species. Experimental stiffness values were significantly different across species and specimen-specific models required species-specific linear relationship between image grayscale and elastic modulus. A good agreement between in vitro and in silico values was achieved for all species, reinforcing the generality of the developed methodology.

Meniscus tear surgery and meniscus replacement

OBJECTIVE

the menisci are easily injured and difficult to repair. The aim of this study was to analyze the current state of meniscal surgery aimed at preserving morphology and conserving the biomechanics of the knee to prevent joint degeneration.

METHODOLOGY

a search of the electronic medical literature database Medline was conducted, from http://www.ncbi.nlm.nih.gov/pubmed. The search was not limited by language. Candidate articles were identified by searching for those that included the keywords meniscus, surgery, suture, implant, allograft. The limits were included for clinical research and clinical trials. Basic research was not included. The studies selected were evaluated and classified in three different categories: basic science, reconstruction (suture and meniscectomy) and implants (scaffolds and allograft).

RESULTS

the consequences of meniscectomy performed at a young age can lead to a joint cartilage degeneration twenty years later. There are few surgical options for the repair of meniscal injuries in order both to preserve the meniscus and to ensure the long term survival of the knee joint, meniscectomy, repair, suturing the tear, or reconstruction, when a meniscal allograft or synthetic substitute is used to replace the meniscus, but the biomechanical properties of the native meniscus are not reproduced entirely by the scaffolds that exist today.

CONCLUSION

therapies that successfully repair or replace the meniscus are therefore likely to prevent or delay osteoarthritis progression.

The protective effect of meniscus allograft transplantation on articular cartilage: a systematic review of animal studies

Despite widespread reporting on clinical results, the effect of meniscus allograft transplantation on the development of osteoarthritis is still unclear. The aim of this study was to systematically review all studies on the effect of meniscus allograft transplantation on articular cartilage in animals. Pubmed and Embase were searched for original articles concerning the effect of meniscus allograft transplantation on articular cartilage compared with both its positive (meniscectomy) and negative (either sham or non-operated) control in healthy animals. Outcome measures related to assessment of damage to articular cartilage were divided in five principal outcome categories. Standardized mean differences (SMD) were calculated and pooled to obtain an overall SMD and 95% confidence interval. 17 articles were identified, representing 14 original animal cohorts with an average timing of data collection of 24 weeks [range 4 weeks; 30 months]. Compared to a negative control, meniscus allograft transplantation caused gross macroscopic (1.45 [0.95; 1.95]), histological (3.43 [2.25; 4.61]) damage to articular cartilage, and osteoarthritic changes on radiographs (3.12 [1.42; 4.82]). Moreover, results on histomorphometrics and cartilage biomechanics are supportive of this detrimental effect on cartilage. On the other hand, meniscus allograft transplantation caused significantly less gross macroscopic (-1.19 [-1.84; -0.54]) and histological (-1.70 [-2.67; -0.74]) damage to articular cartilage when compared to meniscectomy. However, there was no difference in osteoarthritic changes on plain radiographs (0.04 [-0.48; 0.57]), and results on histomorphometrics and biomechanics did neither show a difference in effect between meniscus allograft transplantation and meniscectomy. In conclusion, although meniscus allograft transplantation does not protect articular cartilage from damage, it reduces the extent of it when compared with meniscectomy.

Meniscal allograft transplantation. Part 2: systematic review of transplant timing, outcomes, return to competition, associated procedures, and prevention of osteoarthritis

PURPOSE

To provide a systematic review of the literature regarding five topics in meniscal allograft transplantation (MAT): optimal timing for transplantation, outcomes, return to competition, associated procedures, and prevention of osteoarthritis.

METHODS

A systematic literature search was conducted using the PubMed (MEDLINE), ScienceDirect, and EBSCO-CINAHL databases. Articles were classified only in one topic, but information contained could be reported into other topics. Information was classified according to type of study (animal, in vitro human, and in vivo human) and level of evidence (for in vivo human studies). Specific inclusion criteria were determined for the outcomes and prevention of osteoarthritis topics.

RESULTS

Twenty-four studies were finally included: two optimal timing, seven outcomes, three return to competitive sport, 16 MAT and associated procedures, and 5 MAT and prevention of osteoarthritis (some studies were categorized in more than one topic). These studies corresponded to 2 animal studies and 31 in vivo human studies (1 level II, 1 level III, and 29 level IV).

CONCLUSIONS

The principal conclusions were as follows: (a) there is no evidence to support that MAT has to be performed at the same time or immediately after meniscectomy to prevent development of postmeniscectomy syndrome; (b) MAT successfully improves symptoms, function, and quality of life at 7-to-14 years of follow-up (level IV evidence); (c) the overall failure rate (need for knee arthroplasty) is 10-29% at long-term follow-up; (d) MAT allows return to same level of competition in 75-85% of patients at short- to mid-term follow-up (only three studies level IV evidence with small sample size); (e) associated cartilage procedures or anterior cruciate ligament reconstruction to MAT does not worsen the results; (f) MAT may prevent progression of cartilage damaged at long-term follow-up, but may not prevent degeneration in previously healthy cartilage.

LEVEL OF EVIDENCE

Systematic review of level II-IV studies, Level IV.

Primary and secondary restraints of human and ovine knees for simulated in vivo gait kinematics

Knee soft tissue structures are frequently injured, leading to the development of osteoarthritis even with treatment. Understanding how these structures contribute to knee function during activities of daily living (ADLs) is crucial in creating more effective treatments. This study was designed to determine the role of different knee structures during a simulated ADL in both human knees and ovine stifle joints. A six degree-of-freedom robot was used to reproduce each species' in vivo gait while measuring three-dimensional joint forces and torques. Using a semi-randomized selective cutting method, we determined the primary and secondary structures contributing to the forces and torques along and about each anatomical axis. In both species, the bony interaction, ACL, and medial meniscus provided most of the force contributions during stance, whereas the ovine MCL, human bone, and ACLs of both species were the key contributors during swing. This study contributes to our overarching goal of establishing functional tissue engineering parameters for knee structures by further validating biomechanical similarities between the ovine model and the human to provide a platform for measuring biomechanics during an in vivo ADL. These parameters will be used to develop more effective treatments for knee injuries to reduce or eliminate the incidence of osteoarthritis.

The influence of nonanatomical insertion and incongruence of meniscal transplants on the articular cartilage in an ovine model

BACKGROUND

Adequate size matching and anatomically correct positioning must be recognized as essential factors influencing the outcome of meniscal transplantation.

HYPOTHESIS

Nonanatomical insertion and incongruence of meniscal transplants has an influence on the development of degenerative changes.

STUDY DESIGN

Controlled laboratory study.

METHODS

Ten female sheep were used for this animal study. Both knees (N = 20) were divided into 3 groups, subjected to either meniscectomy (group I; n = 10), to a medial meniscal autograft transplantation with a nonanatomical insertion of the anterior and posterior horn (group II; n = 5), or a meniscal autograft transplantation from the opposite knee as an incongruent meniscal autograft (group III; n = 5). After 6 months, radiographic (Fairbank's criteria), macroscopic (Jackson score), and histological evaluation by light microscopy (Mankin score) and scanning electron microscopy of the articular cartilage was performed.

RESULTS

All applied evaluation methods demonstrated that nonanatomical insertion of meniscal transplants resulted in the highest amount of degenerative cartilage changes. The histological assessment even revealed a significantly enlarged cartilage damage for the non-anatomic-positioned meniscal transplants in relation to the meniscectomized knees. Furthermore, the incongruent meniscal transplants demonstrated a significantly better cartilage situation than nonanatomically inserted meniscal transplants.

CONCLUSION

The histological evaluation demonstrated clearly that a nonanatomically inserted meniscal transplant leads to degenerative cartilage changes that are worse than that after meniscectomy.

CLINICAL RELEVANCE

Precise anatomic positioning is mandatory for the potential chondroprotective effect of meniscal transplants.

Knee cartilage thickness measurements using MRI: a 4(1/2)-month longitudinal study in the meniscectomized guinea pig model of OA

OBJECTIVE

The aim of this study was to follow, over a 4(1/2)-month period, the medial tibia cartilage thickness on a meniscectomy (MNX) guinea pig osteoarthritis (OA) model and to compare with control animals, using three-dimensional high-resolution magnetic resonance imaging (3D HR-MRI).

METHODS

MRI experimentations were performed in vivo at 7 T on guinea pig knee joints. 3D HR-MR images were acquired in 60 controls (SHAM) and 45 osteoarthritic animals (MNX) at four time-points (15, 45, 90 and 135 days) after surgery. Medial tibial cartilage thickness was measured from MRI images using in-house dedicated 3D software followed by a statistical analysis. At each time-point 15 SHAM and 15 MNX animals were sacrificed for histomorphometric assessments.

RESULTS

No significant difference of mean cartilage thickness between the groups was found at early stage (D45) using MRI; however, significant differences were found between the groups at D90 (P<0.001) and D135 (P<0.001). Histomorphometry data confirmed the pathological status of the animals and was well correlated with MRI at D15 (r=0.79, P<0.01), D45 (r=0.67, P<0.01), and D135 (r=0.39, P<0.05) for SHAM, and at D45 (r=0.63, P<0.01), and D135 (r=0.81, P<0.01) for MNX.

CONCLUSION

Medial tibial cartilage measurement based on HR-MR images enables the monitoring of longitudinal cartilage thickness changes. This technique showed significant differences between SHAM and MNX as from D90 after surgery. It could be used as a noninvasive and reproducible tool to monitor therapeutic response in this OA model.

Meniscal allograft transplantation in the sheep knee: evaluation of chondroprotective effects

BACKGROUND

Early protection of articular cartilage, before degenerative changes appear on radiographs, should result in better long-term results, but scientific evidence regarding the effectiveness of meniscal transplantation is lacking.

PURPOSE

To assess the chondroprotective effects of a new meniscal allograft transplantation animal model and evaluate a magnetic resonance imaging parameter, T2 mapping, in articular cartilage after meniscectomy and meniscal transplantation.

STUDY DESIGN

Controlled laboratory study.

METHODS

Forty-five skeletally mature sheep were placed into 3 surgical groups: lateral meniscectomy (n = 24), meniscal allograft transplant (n = 17), and sham (n = 4). Animals were sacrificed at 2, 4, or 12 months. Cartilage was assessed by gross inspection, magnetic resonance imaging, T2 mapping, biomechanical testing, and semiquantitative histologic analysis.

RESULTS

There were no differences between the sham operation and nonoperated control limbs. Compared with control limbs, meniscectomy resulted in significant increases in cartilage degeneration by all objective criteria (P < .01). Compared with meniscectomy, meniscal allograft transplantation resulted in significant decreases in cartilage degeneration (P < .02). There were significant correlations between T2-mapping data and all other traditional outcomes measures (P < .05, r(2) = 0.37-0.67). Compared with the nonoperated control limbs, allograft transplants demonstrated no significant differences at 2 months in any category, except magnetic resonance imaging data. By 4 months, nonoperated control limbs demonstrated significantly less wear compared to allograft limbs in all categories except modified Mankin scores.

CONCLUSION

This model demonstrated significant chondroprotection compared with meniscectomy but demonstrated more cartilage wear at 4 months compared to control limbs. A high degree of allograft cell viability and vascular ingrowth was seen in allograft explants. T2 mapping may provide an accurate noninvasive measure of early cartilage degeneration after meniscectomy, as well as cartilage protection after meniscal allograft transplantation.

CLINICAL RELEVANCE

This study establishes a reliable animal model for meniscal allograft transplantation and provides evidence for the utility of T2 mapping at clinically relevant magnetic resonance imaging field strengths for evaluation of early cartilage degeneration.

Clinical outcomes following isolated lateral meniscal allograft transplantation

PURPOSE

The purpose of our study was to determine the clinical outcomes following isolated lateral meniscal allograft transplantation.

METHODS

Thirty-two patients were retrospectively reviewed following isolated lateral meniscal allograft transplantation. Twenty-five were available for subjective follow-up, and 17 were available for a complete physical examination and postoperative radiographs. The average duration of follow-up was 3.3 years (range, 2 to 6 years), and the average age of the subjects was 30 years (range, 19 to 45 years). In all, 17 patients had bony fixation and 8 had suture fixation of the anterior and posterior horns of the meniscal allograft.

RESULTS

Ninety-six percent of patients believed that their overall function and activity level were improved following surgery. Short Form-36 (SF-36) physical and mental component summary scores for these subjects were higher than age- and sex-matched scores from the US population. Joint space narrowing of the transplanted lateral compartments was not significantly different when compared with the joint space narrowing of the lateral compartment of the contralateral knee. In addition, preoperative and postoperative radiographic joint space measurements of the involved lateral compartment were significantly associated with subjective assessment, symptoms, sports activity score, Lysholm score, and final International Knee Documentation Committee (IKDC) rating at latest follow-up. Finally, patients fixed with the bony technique had significantly better range of motion, according to IKDC criteria at latest follow-up, compared with the suture fixation group.

CONCLUSIONS

Our results suggest that isolated meniscal allograft transplantation can be a beneficial procedure in properly selected symptomatic patients with a lateral meniscus-deficient knee. The data also suggest that earlier meniscal transplantation, before the onset of significant joint space narrowing, may result in improved outcomes. Finally, bony fixation may have a significant advantage over suture fixation, particularly with regard to knee range of motion.

LEVEL OF EVIDENCE

Level IV, therapeutic case series.

Meniscal allograft transplantation

Meniscal allograft transplantation is a reasonable treatment option for the young patient with symptomatic meniscal deficiency. Although clinical results are promising, in most studies only mixed procedures have been performed, with short- or medium-term follow-up. Important potential prognostic factors include patient selection, severity of degenerative changes, limb stability and alignment, graft sizing and processing methods, graft placement, and graft fixation. The use of meniscal allograft transplantation should be considered a salvage operation for the difficult clinical dilemma of meniscal deficiency in young patients. Nonetheless, in carefully selected patients, this procedure can predictably relieve compartmental symptoms, and, in conjunction with anterior cruciate ligament reconstruction, restore knee stability. In addition, the partial restoration of meniscal function provided by this procedure may slow the degenerative arthritic process.

Long-term results after combined medial meniscal allograft transplantation and anterior cruciate ligament reconstruction: minimum 8.5-year follow-up study

PURPOSE

The purpose of this study was to evaluate the long-term results after medial meniscal allograft transplantation combined with anterior cruciate ligament (ACL) reconstruction.

TYPE OF STUDY

Retrospective clinical outcome study.

METHODS

Between 1990 and 1992, 9 medial meniscal allograft transplantations were performed in symptomatic knees with a previous total or near-total medial meniscectomy. One of the 9 patients required transplant removal postoperatively because of a presumed low-grade infection versus immune reaction. The remaining 8 patients were evaluated, with an average follow-up time of 9.7 years. All 8 patients underwent an ACL reconstruction. One patient also had a staged high tibial osteotomy.

RESULTS

All 8 patients were evaluated with the standard International Knee Documentation Committee (IKDC) form with no normal scores: 1 had a nearly normal score, 4 had abnormal scores, and 3 severely abnormal scores. The IKDC symptoms evaluation produced 2 normal scores, 5 nearly normal scores, and 1 abnormal score. The IKDC function test showed 5 normal scores, 1 nearly normal score, and 2 abnormal scores. Six of the 8 patients were extremely pleased with the function of the knee and were active in recreational sports. All 8 patients would recommend the procedure to a friend and would undergo the procedure again given similar circumstances.

CONCLUSIONS

The findings of this study agree with several other studies with shorter follow-up times that medial meniscal allograft transplantation can significantly improve knee function in symptomatic medial meniscus-deficient knees. The addition of a ligament-stabilizing procedure probably improved the results in this patient population.

LEVEL OF EVIDENCE

Level IV, Case Series.

Meniscal allograft replacement: a 1-year to 6-year experience

PURPOSE

Progressive degenerative arthritis leading to premature pain and functional loss in the postmeniscectomy state is a well- recognized and debilitating condition. Meniscal allograft replacement may be a suitable, early treatment alternative for this population at risk. The purpose of this study was to examine the potential benefits of meniscal allograft replacement on relieving pain and restoring function.

TYPE OF STUDY

Retrospective clinical review.

METHODS

From 1993 to 1999, 29 menisci were implanted in 28 patients. Of these, 25 patients (26 menisci) were available for review. All patients had a minimum of 12 months of follow-up, with an average of 33 months. Study participants included 17 men and 8 women with primary symptoms of pain or instability at study onset. Eighteen patients had grades I through III Outerbridge chondromalacia changes and 7 demonstrated grade IV changes in the affected compartment. Data were collected using the International Knee Documentation Committee (IKDC), Lysholm II, and Tegner scoring systems as well as a visual analogue scale (VAS) for pain measurement.

RESULTS

Our findings revealed that following meniscal allograft replacement, pain was significantly reduced and function was improved (P <.001). In addition, IKDC scores for activity were reported as normal or nearly normal in 17 subjects and abnormal in 8 participants. Outerbridge grade had a significant impact on final outcome; only 3 of 7 with grade IV changes achieved normal or nearly normal scores versus 14 of 18 in those with lesser Outerbridge changes. Isolated implants fared the same as those combined with an ACL reconstruction. Overall satisfaction reported by the subjects averaged 83%. Ten second-look procedures revealed 5 normal menisci, 3 with shrinkage, and 2 with recurrent tears.

CONCLUSIONS

Earlier results from this population of patients indicated substantial pain relief and improved function. The durability of these early results has not met the test of time for those with exposed subchondral bone. However, statistically significant early and midterm improvements in pain, symptoms, and functional status continue to be noteworthy in the properly selected patient.

Longitudinal characterization of synovial fluid biomarkers in the canine meniscectomy model of osteoarthritis

Damage to the meniscus can lead to posttraumatic osteoarthritis. Early markers of joint injury and tissue disease may be useful in developing and administering clinical treatment. We investigated the effects of total medial meniscectomy on biomarkers measured serially in synovial lavage fluid each month for 3 months. Following meniscectomy in dogs, four biomarkers were evaluated: cartilage oligomeric matrix protein, keratan sulfate epitope (5D4), the 3B3(-) neoepitope of chondroitin-6-sulfate, and the 3B3(+) chondroitinase-generated epitope of chondroitin-6-sulfate. Meniscectomy led to statistically significant elevations of all four biomarkers, with levels peaking at 4 weeks. By 12 weeks, the level of the 5D4 epitope returned to the preoperative baseline level whereas that of cartilage oligomeric matrix protein, 3B3(-), and 3B3(+) remained above the baseline. Concentrations of these biomarkers in the knees not operated on did not change significantly from the baseline. The levels of cartilage oligomeric matrix protein and 3B3(-) relative to 3B3(+) remained constant in all knees. In contrast, the level of 5D4 relative to 3B3(+) declined over time in the knee operated on but remained constant in the knee not operated on. These results demonstrate a quantitative change in the molecular components of synovial fluid after meniscectomy, as well as a qualitative change evinced by an alteration in the relative proportions of these epitopes. Extensive analyses showed a strong correlation between serum levels of 3B3(-) from the femoral and cephalic veins; however, serum 3B3(-) was not correlated with synovial fluid 3B3(-). These findings support the hypothesis that the concentrations of select cartilage biomarkers in synovial fluid are altered following meniscectomy and are promising tools for objectively monitoring the induction of osteoarthritis in this model system.

The protective effects of meniscal transplantation on cartilage. An experimental study in sheep

BACKGROUND

Meniscal loss may result in arthritis. The aim of this study was to establish a simple operative method for meniscal transplantation in a large-animal model and to determine whether meniscal transplantation provides protection of the articular surfaces, whether meniscal allografts have the same protective effect as meniscal autogenous grafts, and whether there is any rejection phenomenon associated with meniscal allografts.

METHODS

Twenty-eight sheep were divided into four study groups, which were treated with (1) a sham operation (four sheep), (2) a meniscectomy (eight sheep), (3) a meniscal autogenous graft (eight sheep), or (4) a meniscal allograft (eight sheep). The meniscal transplant was secured with three suture anchors to the tibia. At four months after the operation, macroscopic and microscopic evaluations of the articular cartilage and the menisci of the sheep knees were performed in a blinded fashion.

RESULTS

The group treated with the sham operation had no cartilage damage and had normal meniscal tissue. The meniscectomies resulted in significant macroscopic and microscopic damage to the articular cartilage in the medial compartment. The mean score (and standard error of the mean) for macroscopic damage to the cartilage in the group treated with the meniscectomy was 6.5+/-0.8 points compared with 3.9+/-0.7 points in the group treated with the autogenous graft and 4.3+/-0.6 points in the group treated with the allograft (p<0.05). The size of the area of damaged articular cartilage was reduced by approximately 50 percent in both groups treated with a meniscal transplant compared with the group treated with the meniscectomy (p<0.05). There were no significant differences between the group treated with the autogenous graft and that treated with the allograft. The histological appearance of the meniscal autogenous grafts was within normal limits. Interestingly, all of the allografts had evidence of fibrinoid degeneration with areas of hypocellularity and cloning of chondroid cells.

CONCLUSIONS

These results suggest that meniscal transplantation provides noticeable although not complete protection against damage to the articular cartilage after a meniscectomy. The meniscal allografts were just as effective in providing this protection as were the meniscal autogenous grafts.

Meniscal substitutes--animal experience

Animal studies have shown that meniscus allografts and tendon autografts generally heal to the capsule, are revascularized and repopulated with host cells. In animals, neither meniscal allografts nor tendon or fat autografts gain the properties of a normal meniscus. Meniscus allografts and tendon autografts are promising as both seem to offer some protection to the cartilage of the tibial plateau. There is no evidence that meniscal transplantation can prevent cartilage degenerative changes, and the long-term effect of meniscal transplantation on articular cartilage remains unknown. Whether cellular repopulation of the meniscal allograft is sufficient to restore its biomechanical properties is unknown. Collagen scaffolds and tissue engineered grafts are still under investigation, showing promising results especially for the former. Viable meniscal allografts should be implanted within 1 to 2 weeks after harvesting, as the production of proteoglycans decreases after 2 weeks.

The surgical anatomy of the stifle joint in sheep

OBJECTIVE

To provide a detailed description of the surgical anatomy of the stifle joint in sheep.

STUDY DESIGN

The results from analysis of cadaveric dissections (14 stifle joints) and stifle radiographs (8 sets of mediolateral and craniocaudal radiographs) are presented.

ANIMALS

Skeletally mature ewes of mixed breeds.

CONCLUSIONS

Although the anatomy of the ovine stifle joint is similar to that of the human knee joint, a number of unique features were identified. These included the presence of the tendon of the m. extensor digitorum longus on the craniolateral aspect of the stifle joint, the absence of a cranial meniscofemoral ligament (ligament of Humphrey) in the caudal joint space, and attachment of the patellar tendon to the cranial pole of the patella (rather than to the distal pole, as in humans). The implications of these differences are discussed with reference to the suitability of the ovine stifle as a surgical model for the human knee joint.