Anterior Cruciate Ligament Biology and Its Relationship to Injury Forces

Removal of the gonads with surgical sterilization results in a loss of negative feedback to the hypothalamus and anterior pituitary. The sustained supraphysiologic luteinizing hormone (LH) concentrations in gonadectomized dogs can significantly alter organ function and even induce neoplastic changes. For example, gonad removal has a profound effect on thyroid function and is reported to be the most significant cause for the development of hypothyroidism in dogs. Thirty percent more gonadectomized dogs develop hypothyroidism compared with intact dogs. Within the canine thyroid, LH receptors are co-localized with thyroid stimulating hormone (TSH) receptors. Continuous LH receptor activation in gonadectomized dogs may interfere TSH receptor function by consuming second messengers involved in G-protein receptor cell signaling, preventing the action of TSH when it binds to its receptor in the thyroid, resulting in hypothyroidism. The incidence of anterior cruciate ligament ruptures is significantly increased following gonad removal independent of breed, sex, weight or body condition. Luteinizing hormone receptors are expressed in the cruciate ligament and continuous LH receptor activation may increase laxity in these ligaments, resulting in joint instability. Both male and female gonadectomized dogs are at a significantly increased risk for lymphoma and hemangiosarcoma. Luteinizing hormone receptors are also abundant in these tissues. Research in four canine hemangiosarcoma cell lines found that LH receptor activation induces cell proliferation. In addition, research in three canine T-cell lymphoma cell lines found that LH receptor activation induces cell proliferation, adhesion, and invasion as well as increases LH receptor expression. Research is needed to determine if LH reducing strategies using gonadotropin releasing hormone agonists will increase remission times in gonadectomized dogs with LH receptor-positive tumors. In conclusion, among the non-reproductive functions of gonads, suppression of LH secretion and resulting LH receptor overexpression appear necessary in maintaining endocrine, musculoskeletal, and anti-neoplastic health.

Understanding the yield and failure mechanisms of ligaments and tendons is important to have a deeper knowledge of their structure and function. Evaluating what are the limits of the human body is also important to prevent injuries in workers, in athletes and the elderly. The tissue yield mechanism was analyzed by modifying and extending a probabilistic model of collagen bundles. Since not usable experimental data are available in the literature, the model and the method were tested using Monte Carlo simulations. The simulations showed many crucial aspects of the model and gave some indications about possible future real validation experiments. The analysis of the correlation between the simulated data, the model ($R^2$) and the Signal-to-Noise-Ratio (SNR) highlighted the most important parameters that affect effectiveness of the described method: number of fibers, elongation step, noise. This analysis also showed that the numerical differentiation algorithms of the data have a key role on the accuracy of the yield assessment. However, the results also showed that the method is able to correctly estimate the elongation break distribution of the fibers of ligaments and tendons.

Articular cartilage, epiphyseal growth plate and tendons have been recognized as targets of fluoroquinolone-induced connective tissue toxicity. The effects of fluoroquinolones on ligament tissues are still unknown. The aim of this study was to investigate the effects of levofloxacin, a typical fluoroquinolone antibiotic drug, on rabbit anterior cruciate ligament (ACL) cells in vitro. Rabbit ACL cells were treated with levofloxacin at different concentrations (0, 14, 28, 56, 112 and 224 μM) and were assessed to determine the possible cytotoxic effects of levofloxacin on ACL cells. Levofloxacin, with concentrations ranging from 28 to 224 μM, induced dose-dependent ACL cell apoptosis. Characteristic markers of programmed cell death and degenerative changes were identified by electron microscopy in the ACL cells treated with 28 μM of levofloxacin. Moreover, levofloxacin significantly increased the mRNA expression of matrix metalloproteinase 3 (MMP-3) and MMP-13 and decreased the expression of tissue inhibitors of metalloproteinase 1 (TIMP-1) in a concentration-dependent manner; TIMP-3 and collagen type I alpha 1 (Col1A1) mRNA expression was not affected. Immunocytochemical analysis indicated that levofloxacin markedly increased the expression of active caspase-3 within a concentration range of 28 to 224 μM, whereas a clear-cut decrease in Col1A1 expression was found with levofloxacin treatment concentrations of 112 and 224 μM, compared to controls. Our data suggest that levofloxacin has cytotoxic effects on ACL cells characterized by enhanced apoptosis and decreased extracellular matrix, which suggest a potential adverse effect of fluoroquinolones.

Anterior cruciate ligaments (ACLs) can rupture with simple movements, suggesting that structural changes in the ligament may reduce the loading capacity of the ligament. We aimed to investigate if proteoglycan and collagen levels were different between ruptured and non-ruptured ACLs. We also compared changes in ruptured tissue over time.

During arthroscopic knee reconstruction surgery 24 ruptured ACLs were collected from participants (10 females; 14 males; mean age 24 years). Four non-ruptured ACLs were obtained from participants undergoing total knee replacement surgery (one female, three males; mean age 66 years). Western blot analysis was used to characterise core proteins of aggrecan, versican, decorin and biglycan and glycosaminoglycan assays were also conducted. Collagen levels were measured by hydroxyproline (OHPr) assays.

Significantly lower levels of collagen, were found in ruptured ACL compared to non-ruptured ACL (p = 0.004). Lower levels of both small and large proteoglycans were found in ruptured than non-ruptured ACLs. No correlation was found between time since rupture and proteoglycan or collagen levels.

Ruptured ACLs had less collagen and proteoglycans than non-ruptured ACLs. These changes indicate either extracellular matrix protein levels were reduced prior to rupture or levels decreased immediately after rupture. It is possible that the composition and structure of ACLs that rupture are different to normal ACLs, potentially reducing the tissue's ability to withstand loading. An enhanced understanding of the aetiology of ACL injury could help identify individuals who may be predisposed to rupture.