Mechanical Loading of Articular Cartilage Reduces IL-1-Induced Enzyme Expression.

OBJECTIVE: Exposure of articular cartilage to interleukin-1 (IL-1) results in increased synthesis of matrix degrading enzymes. Previously mechanical load applied together with IL-1 stimulation was found to reduce aggrecan cleavage by ADAMTS-4 and 5 and MMP-1, -3, -9, and -13 and reduce proteoglycan loss from the extracellular matrix. To further delineate the inhibition mechanism the gene expression of ADAMTS-4 and 5; MMP-1, -3, -9, and -13; and TIMP-1, -2, and -3 were measured. DESIGN: Mature bovine articular cartilage was stimulated with a 0.5 MPa compressive stress and 10 ng/ml of IL-1α for 3 days and then allowed to recover without stimulation for 1 additional day. The media was assayed for proteoglycan content on a daily basis, while chondrocyte gene expression (mRNA) was measured during stimulation and 1 day of recovery. RESULTS: Mechanical load alone did not change the gene expression for ADAMTS, MMP, or TIMP. IL-1 caused an increase in gene expression for all enzymes after 1 day of stimulation while not affecting the TIMP levels. Load applied together with IL-1 decreased the expression levels of ADAMTS-4 and -5 and MMP-1 and -3 and increased TIMP-3 expression. CONCLUSIONS: A mechanical load appears to modify cartilage degradation by IL-1 at the cellular level by reducing mRNA.

Mechanical load inhibits IL-1 induced matrix degradation in articular cartilage.

OBJECTIVE: Osteoarthritis is a disease process of cellular degradation of articular cartilage caused by mechanical loads and inflammatory cytokines. We studied the cellular response in native cartilage subjected to a mechanical load administered simultaneously with an inflammatory cytokine interleukin-1 (IL-1), hypothesizing that the combination of load and cytokine would result in accelerated extracellular matrix (ECM) degradation. METHODS: Mature bovine articular cartilage was loaded for 3 days (stimulation) with 0.2 and 0.5 MPa stresses, with and without IL-1 (IL-1alpha, 10 ng/ml), followed by 3 days of no stimulation (recovery). Aggrecan and collagen loss were measured as well as aggrecan cleavage using monoclonal antibodies AF-28 and BC-3 for cleavage by aggrecanases (ADAMTS) and matrix metalloproteinases (MMPs), respectively. RESULTS: Incubation with IL-1 caused aggrecan cleavage by aggrecanases and MMPs during the 3 days of stimulation. A load of 0.5 MPa inhibited the IL-1-induced aggrecan loss while no inhibition was found for the 0.2 MPa stress. There was no collagen loss during the treatments but upon load and IL-1 removal proteoglycan and collagen loss increased. Load itself under these conditions was found to have no effect when compared to the unloaded controls. CONCLUSIONS: A mechanical load of sufficient magnitude can inhibit ECM degradation by chondrocytes when stimulated by IL-1. The molecular mechanisms involved in this process are not clear but probably involve altered mechanochemical signal transduction between the ECM and chondrocyte.

Nondegradable hydrogels for the treatment of focal cartilage defects.

Nondegradable materials have long been suggested for the treatment of articular cartilage defects; however, the mechanics of the implant/tissue system necessary to ensure long-term function are unknown. The objective of this study was to explore the performance of nondegradable hydrogel implants in cartilage defects. Our hypothesis was that the structural integrity of the implant and surrounding tissue would be influenced by the compressive modulus of the material used, and that superior results would be obtained with the implantation of a more compliant material. Poly(vinyl alcohol)-poly(vinyl pyrrolidone) hydrogel implants of two different moduli were implanted into osteochondral defects in a rabbit model. Six-month postoperative histological and mechanical data were used to assess the wear and fixation of the implants. The compliant implants remained well fixed and a thin layer of soft tissue grew over the surface of the implants. However, gross deformation of the compliant implants occurred and debris was evident in surrounding bone. The stiffer implants were dislocated from their implantation site, but with no accompanying evidence of debris or implant deformation. Our hypothesis that superior results would be obtained with implantation of a more compliant material was rejected; a compromise between the wear and fixation properties dependent on modulus was found.

Effect of compressive strain on cell viability in statically loaded articular cartilage.

Physiological loading of articulating joints is necessary for normal cartilage function. However, conditions of excessive overloading or trauma can cause cartilage injury resulting in matrix damage and cell death. The objective of this study was to evaluate chondrocyte viability within mechanically compressed articular cartilage removed from immature and mature bovine knees. Twenty-three mature and 68 immature cartilage specimens were subjected to static uniaxial confined-creep compressions of 0-70% and the extent of cell death was measured using fluorescent microscopic imaging. In both age groups, cell death was always initiated at the articular surface and increased linearly in depth with increasing strain magnitude. However, most of the cell death was localized within the superficial zone (SZ) of the cartilage matrix with the depth never greater than approximately 500 microm or 25% of the thickness of the test specimen. The immature cartilage was found to have a significantly greater (> 2 times) amount (depth) of cell death compared to the mature cartilage, especially at the higher strains. This finding was attributed to the lower compressive modulus of the immature cartilage in the SZ compared to that of the mature cartilage, resulting in a greater local matrix strain and concomitant cell surface membrane strain in this zone when the matrix was compressed. These results provide further insight into the capacity of articular cartilage in different age groups to resist the severity of traumatic injury from compressive loads.

Fourier transform infrared imaging spectroscopy analysis of collagenase-induced cartilage degradation.

Collagenase treatment of cartilage serves as an in vitro model of the pathological collagen degradation that occurs in the disease osteoarthritis (OA). Fourier transform infrared imaging spectroscopic (FT-IRIS) analysis of collagenase-treated cartilage is performed to elucidate the molecular origin of the spectral changes previously found at the articular surface of human OA cartilage. Bovine cartilage explants are treated with 0.1% collagenase for 0, 15, or 30 min. In situ collagen cleavage is assessed using immunofluorescent staining with an antibody specific for broken type II collagen. The FT-IRIS analysis of the control and treated specimens mirrors the differences previously found between normal and OA cartilage using an infrared fiber optic probe (IFOP). With collagenase treatment, the amide II/1338 cm(-1) area ratio increases while the 1238 cm(-1)/1227 cm(-1) peak ratio decreases. In addition, polarized FT-IRIS demonstrates a more random orientation of the collagen fibrils that correlate spatially with the immunofluorescent-determined regions of broken type II collagen. We can therefore conclude that the spectral changes observed in the collagenase-treated cartilage, and similarly in OA cartilage, arise from changes in collagen structure. These findings support the use of mid-infrared spectral analysis, in particular the minimally invasive IFOP, as potential techniques for the diagnosis and management of degenerative joint diseases such as osteoarthritis.

Fourier transform infrared spectral analysis of degenerative cartilage: an infrared fiber optic probe and imaging study.

A preliminary investigation into the diagnostic potential of an infrared fiber optic probe (IFOP) for evaluating degenerative human articular cartilage is described. Twelve arthritic human tibial plateaus obtained during arthroplasty were analyzed using the IFOP. Infrared spectra were obtained from IFOP contact with articular surface sites visually graded normal or degraded (Collins Scale grade 1 and grade 3, respectively). Comparisons of infrared spectral parameters (peak heights and areas) were made to elucidate spectral indicators of surface degeneration. IFOP spectral analysis revealed subtle but consistent changes between grades 1 and 3 sites. Infrared absorbance bands arising from type II collagen were observed to change with degradation. More degraded tissues exhibited increased amide II (1590-1480 cm(-1))/1338 cm(-1) area ratio (p=0.034) and decreased 1238/1227 cm(-1) peak ratio (p = 0.017); similar changes were seen with Fourier transform infrared imaging spectroscopy (FT-IRIS) analysis. Grades 1 and 3 cartilage showed consistent spectral differences in the amide II, III, and 1338 cm(-1) regions that are likely related to type II collagen degradation that accompanies cartilage degeneration. These results suggest that it may be possible to monitor subtle changes related to early cartilage degeneration, allowing for IFOP use during arthroscopy for in situ determination of cartilage integrity.

Cartilage viability after repetitive loading: a preliminary report.

OBJECTIVE: To assess matrix changes and chondrocyte viability during static and continuous repetitive mechanical loading in mature bovine articular cartilage explants. METHODS: Cartilage explants were continuously loaded either statically or cyclically (0.5 Hz) for 1-72 h (max. stress 1 megapascal). Cell death was assessed using fluorescent probes and detection of DNA strand breakage characteristic of apoptosis. Cell morphology and matrix integrity were evaluated using histology and transmission electron microscopy. RESULTS: Repetitive loading of articular cartilage at physiological levels of stress (1 megapascal) was found to be harmful to only the chondrocytes in the superficial tangential zone (STZ) and depended on the characteristics (static vs cyclic) and duration (1-72 h) of the applied load. The chondrocytes in the middle and deep zone remained viable at all times. Static loads caused cell death at an early time (3 h) as compared with cyclic loads (sinusoidal, 0.5 cycles per s for 6 h). The amount and extent of cell death peaked at 6 h of cyclic loading, and did not change in subsequent experiments run for longer periods of time (up to 72 h). There was no indication of fragmented nuclear DNA but there was evidence of injurious cell death (necrosis) by electron microscopy. Morphological analysis of cartilage repetitively loaded for 24 h showed matrix damage only in the uppermost superficial layer at the articular surface, reminiscent of the early stages of osteoarthritis. CONCLUSIONS: Cell death in mature cartilage explants occurred after 6 hours of continuous repetitive load or 3 h of static load. Cell death was directly related to the mechanical load, as control (free-swelling) explants remained viable at all times. The excessive, repetitive loading conditions imposed are not physiological, and demonstrate the deleterious effects of mechanical overload resulting in morphological and cellular damage similar to that seen in degenerative joint disease.

FTIR microscopic imaging of collagen and proteoglycan in bovine cartilage.

Articular cartilage, a connective tissue that provides resistance to compressive forces during joint movements, has not been examined in detail by conventional Fourier transform infrared (FTIR) spectroscopy, microspectroscopy (FTIRM), or imaging (FTIRI). The current study reports FTIRM and FTIRI analyses of normal bovine cartilage and identifies the specific molecular components of cartilage that contribute to its IR spectrum. FTIRM data acquired through the superficial, middle, and deep zones of thin sections of bovine articular cartilage showed a variation in intensities of the absorbance bands that arise from the primary nonaqueous components of cartilage, collagen, and proteoglycan (primarily aggrecan) and thus reflected the differences in quantity of these specific components. The spectra of mixtures of model compounds, which had varying proportions of type II collagen and aggrecan, were analyzed to identify spectral markers that could be used to quantitatively analyze these components in cartilage. Collagen and aggrecan were then imaged by FTIRI based on markers found in the model compounds. Polarization experiments were also performed to determine the spatial distribution of the collagen orientation in the different zones of cartilage. This study provides a framework in which complex pathological changes in this heterogeneous tissue can be assessed by IR microscopic imaging.

Proton spin-spin relaxation study of molecular dynamics and proteoglycan hydration in articular cartilage.

Spin-spin relaxation of proton magnetization in natural and deuterated articular cartilage is reported over a range of hydration. Information about macromolecular dynamics is deduced and a hydration stabilized macromolecular regime identified. There is good correspondence between NMR results and cartilage stoichiometry. A new measure for hydration of proteoglycans is found.

Effect of serum and platelet-derived growth factor on chondrocytes grown in collagen gels.

In this in vitro study, cell proliferation, viability, and morphology; proteoglycan (PG) synthesis; and gel contraction were assessed over a 15-day period (on days 3, 6, 9, 12, and 15) for mature bovine chondrocytes cultured in collagen gels. The environment within the gel was varied by changing the concentration of fetal bovine serum (1% and 10%) and platelet-derived growth factor-BB (PDGF; 0, 10, 50, 100 ng/ml) within the gel and incubation media. Our results showed that the amount of serum or PDGF added to the gels had no effect on cell viability, with >95% of cells remaining alive throughout the experiment. There was a significant increase in cell number over time in all groups, with a higher rate of cell proliferation in gels containing 10% serum and higher concentrations of PDGF. In addition, the amount of serum significantly affected gel contraction with or without PDGF. Gels containing 10% serum contracted on day 10-12, while none of the gels containing 1% serum contracted over the course of the experiment. The PG content within each gel increased with incubation time only for the gels containing 1% serum, and 10 or 100 ng/ml of PDGF. However, on a per cell basis, there was no change in the PG content with time when only serum was used and a significant decrease in the rate of PG production with the addition of PDGF (9.1-27.8 pgPG/cell/day). Cell morphology was also affected by PDGF, with the cells becoming more spindle shaped. Cell alignment within the gels appeared to be most affected by gel contraction. Collagen gels can act as cell carriers for the purpose of tissue engineering. These gels provide a three-dimensional environment in which chondrocytes can proliferate and produce matrix. We have shown how this environment can be controlled to affect gel contraction, rates of cell growth and PG production, and cellular morphology while maintaining cell viability. This information will be useful in determining the conditions in which chondrocytes can be grown within collagen gels and combined with cytokines to create an ideal tissue construct.

Effect of impact load on articular cartilage: cell metabolism and viability, and matrix water content.

Significant evidence exists that trauma to a joint produced by a single impact load below that which causes subchondral bone fracture can result in permanent damage to the cartilage matrix, including surface fissures, loss of proteoglycan, and cell death. Limited information exists, however, on the effect of a varying impact stress on chondrocyte biophysiology and matrix integrity. Based on our previous work, we hypothesized that a stress-dependent response exists for both the chondrocyte's metabolic activity and viability and the matrix's hydration. This hypothesis was tested by impacting bovine cartilage explants with nominal stresses ranging from 0.5 to 65 MPa and measuring proteoglycan biosynthesis, cell viability, and water content immediately after impaction and 24 hours later. We found that proteoglycan biosynthesis decreased and water content increased with increasing impact stress. However, there appeared to be a critical threshold stress (15-20 MPa) that caused cell death and apparent rupture of the collagen fiber matrix at the time of impaction. We concluded that the cell death and collagen rupture are responsible for the observed alterations in the tissue's metabolism and water content, respectively, although the exact mechanism causing this damage could not be determined.

Thermal modification of collagen.

Shoulder capsular shrinkage has recently been proposed as a therapeutic modality in a select group of patients with instability. Basic science research studying the mechanism of collagen shrinkage and the effect of shrinkage on the tissue's mechanical properties is essential to define the ideal process by which to achieve optimal tissue shrinkage. Tissue shrinkage is a function of both time and temperature. This relationship was studied, and a model was derived to describe the relationship mathematically. Tissue shrinkage rate was extremely sensitive to temperature changes. The purpose of this study, was to shrink collagenous tissue thermally and then to measure the mechanical property changes as a function of tissue shrinkage. Uniaxial tensile testing of normal and heat-shrunken bovine tendon was carried out, and a model was developed to express the relationship between shrinkage and mechanical properties. We found that the mechanical properties decreased with increasing shrinkage, and that the maximal allowable shrinkage before significant material property changes occurred was between 15% to 20%. Ultrastructural analysis with transmission electron microscopy showed denaturation of the collagen fibrillar structure and provided direct support for the observed material changes.

Effect of joint compression on inferior stability of the glenohumeral joint.

UNLABELLED: To determine the relative importance of negative intraarticular pressure, capsular tension, and joint compression on inferior stability of the glenohumeral joint we studied 17 fresh, normal adult cadaver shoulders using a "3 degrees of freedom" shoulder test apparatus. Translations were measured in intact and vented shoulders while a 50-N superior and inferior directed force was applied to the shoulder. Three different joint compressive loads (22 N, 111 N, 222 N) were applied externally. Tests were performed in 3 positions of humeral abduction in the scapular plane (0 degree, 45 degrees, 90 degrees) and in 3 positions of rotation (neutral, maximal internal, and maximal external). After tests of the intact and vented shoulder, the glenohumeral ligaments were sectioned and tests were repeated. With minimal joint compression of 22 N, negative intraarticular pressure and capsular tension limited translation of the humeral head on the glenoid. Increasing the joint compressive load to 111 N resulted in a reduction of mean inferior translation from 11.0 mm to 2.0 mm at 0 degree abduction, from 21.5 mm to 1.4 mm at 45 degrees abduction, and from 4.5 mm to 1.2 mm at 90 degrees abduction. With a compressive load of 111 N, venting the capsule or sectioning of glenohumeral ligaments had no effect on inferior stability. CLINICAL RELEVANCE: Glenohumeral joint compression through muscle contraction provides stability against inferior translation of the humeral head, and this effect is more important than negative intraarticular pressure or ligament tension.

Effects of scalpel, electrocautery, and CO2 and KTP lasers on wound healing in rat tongues.

OBJECTIVE: Evaluate wound healing of incisions created by the scalpel, electrocautery, CO2 laser, and potassium titanyl phosphate (KTP) laser in the upper aerodigestive tract in an animal model. STUDY DESIGN: Prospective randomized study in an animal model. METHODS: Postoperative oral intake, histologic depth of injury, and tensile mechanical strength were measured in rat tongues after creating incisions using a scalpel, electrocautery, CO2 laser, and KTP laser. An unpaired, two-tailed Student's t-test was used to compare results between the experimental groups. RESULTS: Oral intake, indirectly assessed by postoperative weight loss, by the third postoperative day was significantly decreased in the electrocautery (P = 0.004), CO2 laser (P = 0.001), and KTP laser (P = 0.0001) groups as compared with the scalpel group. The depth of the wound healing, as assessed by histologic examination, was successively greater for the scalpel (75 +/- 13 microm), electrocautery (110 +/- 10 microm), CO2 laser (145 +/- 10 microm), and KTP laser (195 +/- 23 microm) groups. However, this difference was only statistically significant for the CO2 laser (P = 0.006) and KTP laser (P = 0.01) groups relative to the scalpel group. Wounds created by the KTP laser had the lowest strength (76.5 +/- 6.9 kPa) as compared with the CO2 laser (156 +/- 28.4 kPa), electrocautery (153 +/- 15.7 kPa), and scalpel groups (249 +/- 61.8 kPa). This difference was only statistically significant for the KTP laser group (P = 0.02) when compared with the scalpel group. CONCLUSIONS: Wounds created in the upper aerodigestive tract of rats by scalpels result in the least postoperative weight loss, tissue destruction, and decrease in tensile strength, whereas wounds created by the KTP laser demonstrated a significantly greater postoperative weight loss, depth of wounding, and decrease in tensile strength.

Diffusive properties of immature articular cartilage.

The diffusive properties of immature bovine articular cartilage were determined using two different-sized, uncharged solutes (glucose 180 Da, and dextran 10k Da). Radioactively tagged glucose and dextran were diffused into the cartilage for transport times of 5, 15, and 60 min, and the diffusion and partition coefficients were calculated by fitting the experimental data to a one-dimensional diffusion model. The diffusion and partition coefficients for the two solutes averaged 6.08 +/- 2.19 and 5.09 +/- 2.51 (x 10(-6) cm2/s) and 0.712 +/- 0.149 and 0.615 +/- 0.120, respectively. Both coefficients were significantly greater for glucose compared to the larger dextran. While no statistical differences could be found in the diffusive properties of these solutes in immature cartilage compared to their diffusive properties in mature cartilage, there was some evidence that the larger dextran solute might diffuse faster in the earlier time periods. Finally, the bulk fluid contents between the two types of cartilage were not different even though the immature tissue was significantly thicker (1.6 times) than the mature tissue. Our results indicate that the solute diffusion properties of articular cartilage, at least with respect to uncharged solutes, do not change during skeletal maturation.

Continuous cyclic load reduces proteoglycan release from articular cartilage.

OBJECTIVE: To study the effect of a continuous cyclic mechanical load on the release of newly synthesized proteoglycans (PGs) from mature bovine articular cartilage. METHODS: Viable cartilage explants were continuously loaded with 1 MPa cyclic stress at 1 Hz frequency for 24 h, and the release of labeled (35SO4) PGs measured before, during and after application of the compressive load. To separate the effect of active chondrocyte catabolism from that of passive PG release, PG release in live explants, with and without protease inhibitors to inhibit PG breakdown, was compared to PG release in explants whose chondrocytes were killed prior to loading. RESULTS: In live explants, a continuous cyclic load significantly reduced PG release by as much as 50% compared to unloaded explants. In killed explants which were unloaded, the PG release increased five to 10 times, while a cyclic load reduced PG release to that found in viable, loaded explants. Twenty-four hours after load removal PG release in all loaded explants returned (increased) to that of the unloaded explants. CONCLUSIONS: These results indicate that PG release from the cartilage matrix is inhibited by continuous cyclic mechanical loading, independent of cellular metabolism, and suggest that a primary mechanism for reducing PG release is by decreasing the interstitial porosity through which the PGs can escape.

Effect of humeral head component size on hemiarthroplasty translations and rotations.

Glenohumeral translation and rotation were measured in 6 grossly normal, fresh frozen shoulder preparations while a manual load was applied to the humerus. The same tests (maximum elevation, total rotation, anterior/posterior (A/P) translation, and inferior translation) were repeated for each shoulder through 8 series: 1 with the shoulder intact, 1 with the shoulder vented, and 6 with progressively larger humeral head components after hemiarthroplasty. There was an inverse linear relation between humeral head component size and all 4 outcome variables. Replacing the native head with a component of equal diameter reduced elevation 20%, rotation 40%, A/P translation 50%, and inferior translation 60% in the vented shoulder. Replacing the native head with a component of equal effective volume decreased elevation 8%, rotation 20%, A/P translation 25%, and inferior translation 40% in the vented shoulder. Increasing humeral head component size decreased rotation, A/P translation, and inferior translation by similar percentages and elevation somewhat less. Humeral head component size is better described in terms of volume than in terms of diameter or offset.

The combined dynamic and static contributions to subacromial impingement. A biomechanical analysis.

Ten human cadaveric shoulders were tested with a dynamic shoulder model simulating physiologic rotator cuff, deltoid, and biceps muscle forces. The combined effect of the muscle forces and acromial structure on subacromial impingement was measured with minimally invasive, miniature pressure transducers. Shoulders with large acromial spurs had significantly greater impingement pressures at the anterolateral acromion in neutral, internal, and external rotation compared with those with flatter acromia. Application of a biceps muscle force reduced anterolateral acromial pressures by 10%. Failure to simulate a supraspinatus force decreased acromial pressure 52% in shoulders with type III acromia in neutral rotation. Without rotator cuff forces applied, the maximum deltoid muscle force required to elevate the arm increased by 17%. Acromial pressures were increased when no rotator cuff forces were applied, but the increases were not significant. After an anterior acromioplasty, pressures decreased by 99% anteriorly. However, failure to achieve a flat surface posteriorly increased pressures in this location, especially with the shoulder in external rotation. Modeling the rotator cuff and deltoid muscle forces demonstrated the importance of the muscular force couple to center the humeral head during elevation of the arm. The inferior forces of the infraspinatus, teres minor, and subscapularis muscles were necessary to neutralize the superior shear force produced by the deltoid and supraspinatus muscles.

Effect of proteoglycan removal on solute mobility in articular cartilage.

Transport of nutrients, cytokines, pharmacologic agents, and matrix components through articular cartilage is critical for the viability and structural integrity of the tissue. To understand the role of the extracellular matrix in regulating this process, we measured the diffusivity of three uncharged solutes of different molecular size (glucose, MW 180; inulin, MW 5000; dextran, MW 70,000) into intact cartilage and cartilage that had its proteoglycan (PG) component removed. Solute diffusivity was measured by performing transient (nonsteady state) one-dimensional diffusion tests using radiolabelled solutes. Compared to intact cartilage, the diffusivity of glucose was unchanged after PG removal, inulin was unchanged but dextran increased by 1.7 times after 71% PG removal, and both inulin and dextran increased by 1.6 and 2.0 times, respectively, after 93% PG removal. The diffusivities of inulin and dextran were inversely proportional to the PG content. While no change was found in the tissue's bulk fluid content, PG depletion resulted in an increase in fluid content in the upper regions of the tissue and a decrease in the lower regions. These results indicate that in intact tissue small uncharged solutes have free mobility through the inter-molecular and intra-molecular PG volumes, larger molecules have limited intra-molecular mobility, and very large molecules are excluded from the intra-molecular space.

Biomechanical evaluation of the medial collateral ligament of the elbow.

UNLABELLED: Anatomical dissection and biomechanical testing were used to study twenty-eight cadaveric elbows in order to determine the role of the medial collateral ligament under valgus loading. The medial collateral ligament was composed of anterior, posterior, and occasionally transverse bundles. The anterior bundle was, in turn, composed of anterior and posterior bands that tightened in reciprocal fashion as the elbow was flexed and extended. Sequential cutting of the ligament was performed while rotation caused by valgus torque was measured. The anterior band of the anterior bundle was the primary restraint to valgus rotation at 30, 60, and 90 degrees of flexion and was a co-primary restraint at 120 degrees of flexion. The posterior band of the anterior bundle was a co-primary restraint at 120 degrees of flexion and a secondary restraint at 30 and 90 degrees of flexion. The posterior bundle was a secondary restraint at 30 degrees only. The reciprocal anterior and posterior bands have distinct biomechanical roles and theoretically may be injured separately. The anterior band was more vulnerable to valgus overload when the elbow was extended, whereas the posterior band was more vulnerable when the elbow was flexed. The posterior bundle was not vulnerable to valgus overload unless the anterior bundle was completely disrupted. The intact elbows rotated a mean of 3.6 degrees between the neutral position and the two-newton-meter valgus torque position. Cutting of the entire anterior bundle caused an additional 3.2 degrees of rotation at 90 degrees of flexion, where the effect was greatest. CLINICAL RELEVANCE: Physical findings in a patient who has an injury of the anterior bundle may be subtle, and an examination should be performed with the elbow in 90 degrees of flexion for greatest sensitivity. As the anterior bundle is the major restraint to valgus rotation, reconstructive procedures should focus on anatomical reproduction of that structure. Parallel limbs of tendon graft placed from the inferior aspect of the medial epicondyle to the area of the sublimis tubercle will simulate the reciprocal bands of the anterior bundle. Temporary immobilization with the elbow in flexion may relax the critically important anterior band of the reconstruction during healing.