Knotless anchors with sutures external to the anchor body may be at risk for suture cutting through osteopenic bone.

OBJECTIVES: This study evaluated the mechanical performance, under low-load cyclic loading, of two different knotless suture anchor designs: sutures completely internal to the anchor body (SpeedScrew) and sutures external to the anchor body and adjacent to bone (MultiFIX P). METHODS: Using standard suture loops pulled in-line with the rotator cuff (approximately 60°), anchors were tested in cadaveric bone and foam blocks representing normal to osteopenic bone. Mechanical testing included preloading to 10 N and cyclic loading for 500 cycles from 10 N to 60 N at 60 mm/min. The parameters evaluated were initial displacement, cyclic displacement and number of cycles and load at 3 mm displacement relative to preload. Video recording throughout testing documented the predominant source of suture displacement and the distance of 'suture cutting through bone'. RESULTS: In cadaveric bone and foam blocks, MultiFIX P anchors had significantly greater initial displacement, and lower number of cycles and lower load at 3 mm displacement than SpeedScrew anchors. Video analysis revealed 'suture cutting through bone' as the predominant source of suture displacement in cadaveric bone (qualitative) and greater 'suture cutting through bone' comparing MultiFIX P with SpeedScrew anchors in foam blocks (quantitative). The greater suture displacement in MultiFIX P anchors was predominantly from suture cutting through bone, which was enhanced in an osteopenic bone model. CONCLUSIONS: Anchors with sutures external to the anchor body are at risk for suture cutting through bone since the suture eyelet is at the distal tip of the implant and the suture directly abrades against the bone edge during cyclic loading. Suture cutting through bone may be a significant source of fixation failure, particularly in osteopenic bone.Cite this article: Y. Ono, J. M. Woodmass, A. A. Nelson, R. S. Boorman, G. M. Thornton, I. K. Y. Lo. Knotless anchors with sutures external to the anchor body may be at risk for suture cutting through osteopenic bone. Bone Joint Res 2016;5:269-275. DOI: 10.1302/2046-3758.56.2000535.

Surgical menopause initiates molecular changes that do not result in mechanical changes in normal and healing ligaments.

OBJECTIVES: Ligaments which heal spontaneously have a healing process that is similar to skin wound healing. Menopause impairs skin wound healing and may likewise impair ligament healing. Our purpose in this study was to investigate the effect of surgical menopause on ligament healing in a rabbit medial collateral ligament model. METHODS: Surgical menopause was induced with ovariohysterectomy surgery in adult female rabbits. Ligament injury was created by making a surgical gap in the midsubstance of the medial collateral ligament. Ligaments were allowed to heal for six or 14 weeks in the presence or absence of oestrogen before being compared with uninjured ligaments. Molecular assessment examined the messenger ribonucleic acid levels for collagens, proteoglycans, proteinases, hormone receptors, growth factors and inflammatory mediators. Mechanical assessments examined ligament laxity, total creep strain and failure stress. RESULTS: Surgical menopause in normal medial collateral ligaments initiated molecular changes in all the categories evaluated. In early healing medial collateral ligaments, surgical menopause resulted in downregulation of specific collagens, proteinases and inflammatory mediators at 6 weeks of healing, and proteoglycans, growth factors and hormone receptors at 14 weeks of healing. Surgical menopause did not produce mechanical changes in normal or early healing medial collateral ligaments. With or without surgical menopause, healing ligaments exhibited increased total creep strain and decreased failure stress compared with uninjured ligaments. CONCLUSIONS: Surgical menopause did not affect the mechanical properties of normal or early healing medial collateral ligaments in a rabbit model. The results in this preclinical model suggest that menopause may result in no further impairment to the ligament healing process. Cite this article: Bone Joint Res 2015;4:38-44.

The interface of mechanical loading and biological variables as they pertain to the development of tendinosis.

Different tendons are designed to withstand different mechanical loads in their individual environments. Variable physiologic loading ranges and correspondingly different injury thresholds lead to tendon heterogeneity. Also, tendon heterogeneity is evident when examining how different tendons regulate their response to changes in mechanical loading (over- and under-loading). The response of tendons to changes in mechanical loading plays an important role in the induction and progression of tendinosis which is tendon degeneration without inflammation. Tendon overuse injury is likely related to abnormal mechanical loading that deviates from normal mechanical loading in magnitude, frequency, duration and/or direction. Mechanical loading that results in tendon overuse injury can initiate a repair process but, after failed initial repair, non-resolving chronic attempted repair appears to lead to a "smoldering" fibrogenesis. Contributions of regulatory components, including minor components in the "nerve-mast cell-myofibroblast axis", are key features in the development and progression of tendinosis. Hormonal and genetic factors may also influence risk for tendinosis. Further understanding of how tendinosis induction is related to mechanical use/overuse, how tendinosis progression is related to abnormal regulation of attempted repair, and how induction and/or progression are modulated by other risk factors may lead to interventions that mitigate risk and enhance functional repair.

Changes in mechanical loading lead to tendonspecific alterations in MMP and TIMP expression: influence of stress deprivation and intermittent cyclic hydrostatic compression on rat supraspinatus and Achilles tendons.

BACKGROUND: Tendinopathy commonly occurs in tendons with large in vivo loading demands like the Achilles tendon (AT) and supraspinatus tendon (SST). In addition to differences in their local anatomic environment, these tendons are designed for different loading requirements because of the muscles to which they attach, with the AT experiencing higher loads than the SST. One possible factor in the progression of tendinopathy is the interplay between mechanical loading and the regulation of enzymes that degrade the extracellular matrix (matrix metalloproteinases (MMPs)) and their inhibitors (tissue inhibitor of metalloprotienases (TIMPs)). Thus, overuse injuries may have different biological consequences in tendons designed for different in vivo loading demands. AIM: In this study, the tendon-specific regulation of MMP-13, MMP-3 and TIMP-2 expression in rat AT and SST exposed to two different mechanical environments was investigated. METHODS: Rat AT and SST were exposed to stress deprivation (ie, detached from attachments) and intermittent cyclic hydrostatic compression (with attachments intact). Levels of MMP-13, MMP-3 and TIMP-2 mRNA were evaluated in time-zero control, attached, stressdeprived and "compressed" tendons. RESULTS: Stress deprivation led to elevated expression of MMP-13, MMP-3 and TIMP-2 in both tendons, although the magnitude of the increase was greater for the SST than the AT. Intermittent cyclic hydrostatic compression of attached tendons increased expression of MMP-13 in the SST, but not the AT. CONCLUSIONS: The results of this study suggest that stress deprivation may be one contributor to the progression of tendinopathy in AT and SST, where the tendon designed for the lower in vivo loading demand (SST) was the most affected by a change in mechanical loading. The unique upregulation of MMP-13 with hydrostatic compression supports the impingement injury theory for rotator cuff tears.

Strength of medial structures of the knee joint are decreased by isolated injury to the medial collateral ligament and subsequent joint immobilization.

Past studies of the healing of the medial collateral ligament (MCL) in animal models have been conducted over a variety of healing intervals, some as early as 1 week. One concern with testing at early healing intervals is the difficulty in identifying and isolating the tissues that carry load. The purpose of this study was to determine if isolation of the MCL and healing time are critical factors in the assessment of structural strength in this model. Furthermore, the effect of immobilization on these critical factors was investigated. Our approach was to calculate the load-sharing ratio between the MCL and the MCL plus capsule. A 4 mm gap was created in the midsubstance of both hindlimb MCLs of 52 female New Zealand White rabbits (n=104). Of these, 29 rabbits had their right hindlimb pin immobilized (immobilized group), leaving the left hindlimb non-immobilized. Testing was performed at 3 (n=12), 6 (n=22), and 14 (n=24) weeks. The remaining 23 rabbits, which had both limbs non-immobilized (non-immobilized group), were tested at 3 (n=10), 6 (n=12), 14 (n=12), and 40 (n=12) weeks. For both groups, half of the specimens at each healing interval were used to test the MCL alone and half to test the MCL plus capsule, except for 3 week immobilized joints where only the MCL plus capsule was tested. Additionally, MCL (n=12), MCL plus capsule (n=6), and capsule alone (n=5) were tested from normal animals. The load-sharing ratio at MCL failure for the normal joint was 89%, suggesting an MCL-dominated response. For the non-immobilized group, the load-sharing ratio was 24% at 3 weeks of healing, suggesting a capsule-dominated response. At and after 6 weeks of healing, an MCL-dominated response was observed, with the ratio being 68% or greater. Thus, at less than 6 weeks of healing, the structural strength capabilities of the joint may be better represented by the medial structures rather than the isolated MCL. Immobilization delayed the transition from a capsule-dominated response to an MCL-dominated response in this model.

Biomechanical study using fuzzy systems to quantify collagen fiber recruitment and predict creep of the rabbit medial collateral ligament.

In normal daily activities, ligaments are subjected to repeated loads, and respond to this environment with creep and fatigue. While progressive recruitment of the collagen fibers is responsible for the toe region of the ligament stress-strain curve, recruitment also represents an elegant feature to help ligaments resist creep. The use of artificial intelligence techniques in computational modeling allows a large number of parameters and their interactions to be incorporated beyond the capacity of classical mathematical models. The objective of the work described here is to demonstrate a tool for modeling creep of the rabbit medial collateral ligament that can incorporate the different parameters while quantifying the effect of collagen fiber recruitment during creep. An intelligent algorithm was developed to predict ligament creep. The modeling is performed in two steps: first, the ill-defined fiber recruitment is quantified using the fuzzy logic. Second, this fiber recruitment is incorporated along with creep stress and creep time to model creep using an adaptive neurofuzzy inference system. The model was trained and tested using an experimental database including creep tests and crimp image analysis. The model confirms that quantification of fiber recruitment is important for accurate prediction of ligament creep behavior at physiological loads.

Examination of the failure properties of the anterior cruciate ligament during the estrous cycle.

The purpose of this study was to determine whether the mechanical properties of the rat anterior cruciate ligament (ACL) vary when tested in vitro at different stages of the estrous cycle. Sixty female rats were allocated to four groups according to their stage of the estrous cycle: diestrus (n=16), proestrus (n=17), estrus (n=13) and metestrus (n=14). Right hindlimbs were harvested for mechanical testing and left hindlimbs were harvested for immunohistochemical staining to confirm the presence of the estrogen receptor. Results from the first relaxation test showed a significant difference between the estrus and proestrus stage, which was not observed in a second subsequent relaxation test. Likewise, no significant differences were found when comparing failure load and stiffness between the different stages of the estrous cycle. These results suggest that normal physiological fluctuations in estrogen during the estrous cycle did not alter the failure properties of the rat ACL.

Healing ligaments have decreased cyclic modulus compared to normal ligaments and immobilization further compromises healing ligament response to cyclic loading.

Ligaments help maintain joint stability by resisting excessive strain during the repetitive loading experienced during daily activity. Healing ligaments may be less able to fulfill this role, straining more under equivalent loading than normal ligaments. We examined the cyclic stress-strain response of normal and healing ligaments to repetitive low loads (<10% of the normal ligament failure strength). Rabbit medial collateral ligaments (MCLs) were surgically gapped in either a unilateral (right MCL; n=23) or bilateral (right and left MCLs; n=17) fashion with immobilization of the right hindlimb in the bilateral group. These MCL scars were allowed to heal for 3, 6, and 14 weeks and were cyclic creep tested at 2.2, 4.1, and 7.1 MPa, respectively. Creep test stresses were a constant 30% of the failure strength of non-immobilized scars at the different healing intervals. Normal MCLs were creep tested at 4.1 and 7.1 MPa (n=13). The cyclic modulus of the non-immobilized scars was less than that of normal ligaments. The percent increase in modulus during cycling was greater for scars than for normal ligaments, likely related to increased viscous dissipation or material inferiorities in scars. Furthermore, immobilization significantly decreased the ability of scars to resist strain, with a majority of immobilized scars failing during repetitive loading. Such failures were preceded by a reduction in cyclic modulus indicating damage to the healing ligaments that was predictive of eventual total failure. The implications of this study are that joints with healing ligaments may have increased strain in joint structures while they are under stress, potentially leading to joint instability. Although immobilization could be used temporarily to maintain joint stability, remobilization would likely lead to total failure of the healing ligament.

Ligament creep recruits fibres at low stresses and can lead to modulus-reducing fibre damage at higher creep stresses: a study in rabbit medial collateral ligament model.

Ligaments are subjected to a range of loads during different activities in vivo, suggesting that they must resist creep at various stresses. Cyclic and static creep tests of rabbit medial collateral ligament were used as a model to examine creep over a range of stresses in the toe- and linear-regions of the stress-strain curve: 4.1 MPa (n = 7), 7.1 MPa (n = 6), 14 MPa (n = 9) and 28 MPa (n = 6). We quantified ligament creep behaviour to determine if, at low stresses, modulus would increase in a cyclic creep test and collagen fibres would be recruited in a static creep test. At higher creep stresses, a decrease in measured modulus was expected to be a potential marker of damage. The increase in modulus during cyclic creep and the increase in strain during static creep were similar between the three toe-region stresses (4.1, 7.1, 14 MPa). However, at the linear-region stress (28 MPa), both these parameters increased significantly compared to the increases at the three toe-region stresses. A concurrent crimp analysis revealed that collagen fibres were recruited during creep, evidenced by decreased area of crimped fibres at the end of the static creep test. Interestingly, a predominance of straightened fibres was observed at the end of the 28 MPa creep test, suggesting a limited potential for fibre recruitment at higher, linear-region stresses. An additional 28 MPa (n = 6) group had mechanically detectable discontinuities in their stress-strain curves during creep that were related to reductions in modulus and suggested fibre damage. These data support the concept that collagen fibre recruitment is a mechanism by which ligaments resist creep at low stresses. At a higher creep stress, which was still only about a third of the failure capacity, damage to some ligaments occurred and was marked by a sudden reduction in modulus. In the cyclic tests, with continued cycling, the modulus increased back to original values obtained before the discontinuity suggesting that other fibres were being recruited to bear load. These results have important implications for our understanding of how fibre recruitment and stress redistribution act in normal ligament to minimize creep and restore modulus after fibre damage.

Medial collateral ligament autografts have increased creep response for at least two years and early immobilization makes this worse.

Recent evidence has shown that 10-40% of knee joints reconstructed with soft-tissue autografts have a recurrence of abnormal joint laxity over time. One possible explanation is the "stretching out" (or unrecovered creep) of the graft tissue. To test in vitro creep and creep recovery of fresh anatomic ligament autografts in an extra-articular environment, 16 rabbits underwent an orthotopic medial collateral ligament (MCL) autograft procedure to one hindlimb. Three subgroups of animals had either unrestricted cage activity for 1 year (n = 5) or 2 years (n = 5) or pin-immobilization for the first 6 weeks followed by cage activity for the remainder of 1 year (n = 6). Following laxity measurements, to test their creep response, isolated MCL grafts were cyclically and then statically creep tested in vitro at 4.1 MPa, allowed to recover at zero load for 20 min, and finally elongated to failure. Due to differences in cross-sectional area between the grafts and normal MCLs, two normal control groups were tested: stress-matched tested at 4.1 MPa (16.2 N; n = 7) and force-matched tested at 29.1 N (7.1 MPa; n = 6). Ligament grafts had normal laxity but significantly increased creep and decreased creep recovery compared to normal MCLs after I and 2 years of healing (p < 0.0004). Graft failure stress was also significantly less than normal (p < 0.0001). Immobilized grafts had significantly greater creep compared to non-immobilized grafts at 1 year of healing (p < 0.05). These results support previous observations concerning material inferiority of fresh anatomic rabbit MCL autografts, but add the concept that such grafts also have increased potential to creep with either slower or incomplete recovery when subjected to low stresses in vitro. Joint and ligament laxities in situ were normal in this model, however, suggesting either that in vivo MCL graft stresses are lower than those used here in vitro or that these tissues have other mechanisms by which they can recover their functional length in vivo.

Early healing processes of free tendon grafts within bone tunnels is bone-specific: a morphological study in a rabbit model.

In order to function as effective ligament replacements, free tendon grafts must become firmly healed into bone tunnels as soon as possible. We hypothesized that graft incorporation would be bone-specific. Free semitendinosus tendon grafts were inserted into drill holes in a lapine medial collateral ligament reconstruction model; thus, creating tibial and femoral bone-specific incorporation sites. Femur-semitendinosus tendon-tibia complexes were harvested from 26 rabbits for histological analysis at various healing times: 0, 6, 12, or 24 weeks post-surgery. Incorporation and remodeling of the graft in the chondral callus was much more extensive at the cancellous-filled femoral insertion than within the marrow-dominated tibial insertion, suggesting that tendon graft healing may depend on the cancellous bone architecture at the graft site.

Altering ligament water content affects ligament pre-stress and creep behaviour.

The water content of a ligament can be altered by injury and surgical intervention in vivo, and inadvertently or purposely during in vitro tests. We investigated how altering the water content of the rabbit medial collateral ligament (MCL) affected its resulting creep behaviour (defined as an increase in strain from sequential cyclic and static creep tests). The water content of normal MCLs 4) was compared to that of MCLs soaked for 1 h in a sucrose solution (n = 4) or phosphate buffered saline (PBS; n = 8). Sucrose exposure decreased hydration and PBS exposure increased hydration. In addition, soaking in PBS caused a shift in ligament zero (the position where there was 0.1 N of tension on the ligament). Following the same single solution treatment, additional MCLs were creep tested at 4.1 MPa using a load based on the ligament cross-sectional area measured before solution treatment: sucrose (n = 4), PBS new "ligament zero" (n = 5). and PBS old "ligament zero" (n = 6). Normal MCLs were also tested at 4.1 MPa (n = 7) in a humidity chamber that maintained normal ligament water content. Additional MCLs were treated with both solutions in series (n = 12) to examine the reversibility of the mechanical changes caused by single solution treatment. This was the first investigation to show that ligament creep behaviour was clearly affected by the initial state of hydration: creep decreased with decreased hydration and creep increased with increased hydration. Another unique finding was that ligaments with increased hydration had decreased ligament functional length and increased ligament pre-stress. The creep behaviour of these ligaments was decreased if they were loaded from the pre-stressed state compared to the unloaded state. These results suggest that maintenance of physiological water content is important for in vitro mechanical testing of ligaments and controlling the low-load stress state of ligaments in situ.

Early medial collateral ligament scars have inferior creep behaviour.

Recent evidence suggests that ligaments are subject to repetitive loads in vivo. Hence, the creep behaviour (increase in strain under constant or repetitive stress) of ligament scars is of significance, since healing ligaments may elongate permanently over time. A rabbit medial collateral ligament model was used to assess the creep behaviour of healing ligaments at stresses corresponding to 30% of the scar failure strength at 3 (n = 6), 6 (n = 6), and 14 (n = 5) weeks of healing. The stresses for the creep tests of scars (and contralateral controls) were 2.2, 4.1, and 7.1 MPa for the 3, 6, and 14-week healing intervals, respectively. Normal medial collateral ligaments from comparable rabbits were tested at two of the corresponding stresses: 4.1 (n = 7) and 7.1 (n = 6) MPa. Total creep strain-the cumulative increase in strain resulting from serial cyclic and static creep testing-was independent of the order of testing and was compared between scars and controls. Water contents after testing were also quantified. Water contents before testing were assessed for additional animals: six normal animals and three from each healing interval. At 3 weeks of healing, the total creep strain of medial collateral ligament scars was four times greater than that for contralateral controls tested at the same stress. Although there was improvement from 3 to 14 weeks, the total creep strain of scars remained more than two times greater than that of controls at 14 weeks. Scar water content decreased with healing from elevated initial values, possibly contributing to the marginally improved creep response. Comparisons of this deficiency in scar creep with previously published scar abnormalities in the same model suggest that collagen crosslink density, proteoglycan content, soft-tissue flaws, and the combined effect of collagen fibre changes may be mechanistic factors involved in scar creep.

Ligament creep cannot be predicted from stress relaxation at low stress: a biomechanical study of the rabbit medial collateral ligament.

In normal daily activity, ligaments are probably subjected to repeated loading rather than to repeated deformation. The viscoelastic response to repeated loading is creep; this effect has significance for ligament reconstructions, which potentially "stretch out" over time. However, most experimental studies have examined the viscoelastic response to repeated deformation, stress relaxation. We hypothesized that the creep of a ligament could be predicted from its stress-relaxation behaviour. Left and right medial collateral ligaments of eight skeletally mature rabbits were subjected to either creep or stress-relaxation testing under comparable conditions. The time-dependent increase in strain (creep) and reduction in load (relaxation) from the tests were modelled with use of the quasilinear viscoelastic theory and generalized standard linear solid modelling. Ligaments were found to creep distinctly less than would be predicted from relaxation tests. Although the reason for this behaviour remains unknown, we speculate that it is due to the progressive recruitment of collagen fibres during creep.