3. Factors Influencing Damage to the Meniscus

The human knee joint is designed to withstand a lifetime of strenuous activity; however overuse makes the knee heavily prone to injury. Although fracture and dislocation of the knee is rare, a sudden blow or sudden movement that strains the knee beyond its normal movement can cause the joint to fail. The menisci can be subject to traumatic injury due to their central anatomical position and function within the synovial joint, with meniscal tears being one of the most common injuries among sports players [Moran, 2001]. The most fatal tears occur when the meniscus is subject to both compressive strain and a sudden rotation of the tibia with respect to the femur, often occurring among athletes in sports such as football, American football and basketball. Torn meniscal tissue can result in swelling of the knee, severe pain and, in some cases, knee-locking. A tear to the meniscus prevents the cartilage from performing its role within the human anatomy to its full potential, promoting early effects of degenerative disease. With the medial meniscus more prone to large tears through excessive strain, the lateral is more prone to degenerative wear, increasing the cartilage’s vulnerability to further tearing [Moran, 2001]. The medial meniscus is widely known to tear ten times more frequently than the lateral due to it being more strictly attached to the surrounding muscles [Moran, 2001]. Table 3.1, below, displays the common meniscal tears and their frequency of occurrence whilst figure 3.1 provides a diagrammatic representation of their effect upon the geometry of the meniscus.  


Table 3.1 – Common meniscus tears and their frequency of occurrence [Moran, 2001]




Figure 3.1 – Diagrammatic representation of common meniscal tears [Moran, 2001]



The circumferential tensile stress (or ‘hoop’ stress), detailed in section 2.3.5, is thought to be the essential mechanical property of the meniscus and dominate its performance under compressive loading and, ultimately, its failure. Therefore, we can suggest that radial tearing may reduce the capacity of the circumferential collagen fibre bundles to effectively transmit the meniscus’ compressive load. With the torn meniscus unable to perform its designed operation, the susceptibility to degenerative disease will be further enhanced. Figure 3.2 shows an arthroscopic image of a bucket handle and part-longitudinal tear. The open detachment of the central meniscal section can be clearly seen within the bucket handle tear, whilst the rough edges of the part-longitudinal tear are also clearly conveyed.




Figure 3.2 - Arthroscopic image of a bucket handle (A) and part-longitudinal (B) meniscal tear [Rolf, 2007]


Additionally, the meniscus is know to naturally degenerate with age, where is becomes harder, less flexible and more brittle. In this case, degenerative meniscal tears can occur, which leads researchers to the study of the influence of age upon damage to the meniscus. 

3.2. Age
There is vast research to suggest a strong correlation between old age and the effects of osteoarthritis. The decreasing quality of collagen fibres within the meniscus and the body’s increasing inability to produce new collagen fibres with age is thought to be the cause. The fibrils ultimately decrease in flexibility due to an increase in intermolecular cross-linking, increasing the rigidity and brittleness of the fibres until they eventually fail mechanically in fatigue. It is well documented that the collagen content of the meniscus increases from birth to 30 years, remains constant until 80 years and then begins to decline [Moran, 2001]. Therefore, we can suggest that the damaging effects of OA, if untreated before the age of 75-80 years, will become considerably worse as the collagen fibres begin to naturally degrade.


Tensile strength, collagen’s pivotal mechanical property, is thought to increase dramatically with age as the collagen fibre packing increases with cross-linking. This can have significant implications upon the operational qualities of the meniscus. Increases in collagen fibre diameter lead to micro-scarring within the meniscus, thought to be a direct result of increased cross-linking. The composition of the fibres which make up the collagen matrix are also thought to alter with age as the body slowly replaces damaged or old fibres, ultimately changing the functional properties of the tissue.


Conclusively, the collagen fibre matrix which provides the meniscus with its structure and stability degenerates with age. The body’s natural ability to produce fresh collagen protein decreases, having an adverse effect upon the mechanical function of the meniscus. The collagen fibres eventually fail under years of cyclic loading, allowing the degenerative condition, osteoarthritis, to set in as the meniscus can no longer serve its functional purpose effectively.  


Arthritis is a generalised term for the inflammation of a joint, accompanied by pain, swelling and stiffness originating through injury, infection or degenerative changes. Particular to the effects upon the knee, there are three basic types of arthritis: rheumatoid arthritis (RA), osteoarthritis (OA) and post-traumatic arthritis. Common among all three conditions, the knee joint losses its ability to properly support the upper body weight and can result in detrimental effects upon human physical activity, with arthritis known to be the single biggest cause of physical disability in the UK [Moran, 2001]. It is estimated that arthritis affects 1 in 70 people worldwide, with around 150,000 surgical procedures carried out each year in the US due to the damaging effects of arthritis within the knee joint [Enderle et al, 2005]. With post-traumatic arthritis very similar to the effects of OA, the following section details the damaging effects of osteoarthritis and rheumatoid arthritis upon the human knee joint and meniscal tissue.

3.3.1. Osteoarthritis (OA)

Knee osteoarthritis is a degenerative joint disease characterised by the wear and tear of the articular cartilage and chronic irritation of the tibial and patella bone surfaces. OA can cause permanent breakdown of the cartilages that cushion the joint from impact and allow smooth articulation within the joint: the articular cartilage and menisci. A combination of biomechanical forces (enhanced by body weight, activity level and orthopaedic abnormalities) and the natural degenerative process associated with aging contributes to the development of OA. Genetics are also thought to contribute to the effects of OA, such as faults within the genes that control the movement and fixation of joints [Moran, 2001]. As the effects of OA progress, pain, stiffness and limitation of knee motion increase in severity. 


Initial degenerative changes are marked by softening of the articular cartilage (AC), a stage in the process labelled fibrillation, which eventually wears away over time. The surface of the articular cartilage has been found to decrease by 0.6 Mpa, from 7.8 Mpa, during fibrillation [Elfick, 2011]. The bone surfaces within the tibiofemoral and patellofemoral articulations begin to directly grind against each other in the absence of the articular cartilage. The bones begin to erode, with the growth of bony spurs also forming. Tiny shards of bone and cartilage float loosely in the joint space, and at this stage the pain can be severe [Moran, 2001]. The physical effect that osteoarthritis has upon the knee joint can be seen in figure 3.3.


Figure 3.3 – A knee suffering from osteoarthritis [Nuffield Orthopaedic Centre, 2010]


Conventional radiology and arthroscopy are used to determine osteoarthritic effects upon the bones and fibro-cartilage within the knee respectively. Joint space is known to largely narrow in the event of degenerative wear, whilst the meniscus is known to show similar wear characteristics of the articular cartilage due to the absence of the low friction cartilage surface. 

3.3.2. Rheumatoid Arthritis (RA)
Rheumatoid arthritis (RA) is a chronic inflammatory disorder of the joint, affecting adults at any age, but most notably between the ages of 40 and 60. About 1 in 100 people are thought to develop RA at some stage in their life, with 350,000 people in the U.K. suffering from the condition [Arthritis Research (AR) UK, 2010]. Women are around three times more likely to develop RA. The exact causes of rheumatoid arthritis are not yet apparent to medical professionals, although genetics and lifestyle are thought to play a role. RA is more common in people who smoke, who eat a lot of red meat or who drink a lot of caffeine, whilst being less common in people who have a high vitamin C intake.

In rheumatoid arthritis the body's immune system produces inflammation that attacks and damages the tissue and bone of the knee joint, including the surrounding ligaments. Inflammation causes the tough knee capsule to stretch. When the swelling goes down the capsule remains stretched and can no longer hold the joint in its proper position [AR UK, 2010]. RA also thins the cartilage and alters the viscosity of the synovial fluid, affecting the lubrication within the joint. Resultantly, the joint becomes unstable leading to increased mechanical stresses experienced by the meniscus. Figure 3.4 compares a health knee joint (A) and a knee joint subject to rheumatoid arthritis.

Figure 3.4 – Comparing the effects of rheumatoid arthritis (B) with a health knee (A) [AR UK, 2010] 
Black athletes have been dominating sporting & athletic events since the beginning of the nineteenth century, with the successful careers of Joe Lewis (1914-1981), boxer, and Jesse Owens (1913-81), track & field, sparking early interest into the study of sports science [Carrington, 2010]. Thought to have an inherent physical advantage over other human races, sporting success among black athletes in the early nineties was considered innate and natural, whilst white success would require much more diligence and application of the mind. Nowadays, black athletes continue to dominate with 80 percent of National Basketball League (NBA) players, 70 percent of National Football League (NFL) players and around 20 percent of English Premier League players being of African descent [Entine, 2001].

With extensive scientific research proving black and white humans to display distinct differences in physical characteristics such as bone mass and musculoskeletal composition, a detailed analysis of meniscal samples originating from the two races may determine significant differences in the mechanical properties of the menisci tissue. The muscle composition of African descendant humans is thought to be made up from predominantly fast-twitch fibres which contract 10 times quicker than slow twitch fibres - the typical muscle composition of Caucasians [Anderson et al, 2000]. These fast-twitch muscle fibres have allowed Maurice Greene, a black American track & field athlete, to complete the Olympic 100m in less than 10 seconds on over thirty occasions, a barrier that has proved impenetrable for runners of Caucasian, Hispanic or Asian descent [Entine, 2001]. To the knowledge of the researcher, there has been no previous research into the menisci tissue of different races. Therefore, due the limited nature of menisci test samples, the researcher aims solely to introduce the theory that the mechanical properties of human knee menisci and the susceptibility to injury may vary with ethnic origin.

3.5. Gender
It is widely accepted within the medical and sports professions that women are more likely to experience sports-related knee injuries than their male counterparts. Although research studies comparing the incidence of meniscal injuries among males and females are extremely scarce, there is vast research within anterior cruciate ligament (ACL) injury with reference to gender.     With around 50-75 percent of ACL injuries occurring in combination with injury to the meniscus, female athletes are 3-4 times more likely to sustain ACL injures than males [Bauer, 1999]. Research studies throughout the nineties found the ratio of ACL injuries among women sports players compared to men to be as high as 6:1 in soccer and 8:1 in basketball [Swedan, 2001].


Figure 3.5 – Comparison between the pelvic structure and Q angle of males and females [Austin, 2003]


Muscle strength and activation patterns are known to differ between gender, with different skeletal shapes and sizes contributing to slight differences in functional movement between males and females [Bauer, 1999]. Research has proven the anatomy of the human body to vary with gender, with females constituting a wider pelvic structure, narrower femoral surface, increased Q angle (genu valgum) and shorter legs when compared to males [Swedan, 2001]. In addition to the increased pressure the female meniscus must experience due to the narrower femoral surface [P=F/A], the naturally greater Q angle (an abbreviation for the quadriceps femoris muscle angle) predisposes the meniscus to greater stress due to the torsion applied between the femur and the tibia. The Q angle is normally 13o in males and 18o among females, due to their wider pelvis [Austin, 2003], figure 3.5.


Furthermore, research has suggested that hormones may be an influential factor in the female susceptibility to injury compared to males. A lack of circulating androgens during the menstrual cycle discourages the development of large and more powerful muscles, which are designed to act as a protective mechanism for the knee joint [Austin, 2003]. Particular to female sports players and athletes, 80 percent of ACL injuries have been evaluated to occur during non-contact activity such as landing from a jump. Females are thought to be in a more upright position, with less hip and knee flexion and a more outward (valgus) knee angulation when landing, predisposing the athlete to knee injury [Frontera, 2007]. Strength conditioning training has been proven to reduce the varus and valgus torques and the abduction and adduction moments of the knee joint at landing, stabilizing the joint and preventing serious injury [Frontera, 2007].         


With the female race more prone to knee trauma, arthritis is known to affect women around 1.7 times more often than men; therefore total knee surgery can be considered most common among females. Until recently, prosthetic knee replacements came in three distinct sizes (small, medium and large) designed to meet the anatomical features of men more favourably than women. A recent study by Dalury et al (2001), analysed 1970 knee replacements, 920 among women and 592 among men, with a mean age of 69.7 years and at a mean follow up of 7.3 years. The study found minimal differences in the outcome between genders in terms of complications or improvements in knee function, pain score or range of movement. Conventionally selecting the smallest of the three knee prosthetics, there is a distinct danger for overhang of the implant among women due to the narrower femora within the female knee [Dalury et al, 2001]. Nowadays, knee prosthetics have been optimized to fit the characteristic features and shape of a woman’s knee, with a more oblique femoral groove, thinner anterior profile and narrower contour, like the Zimmer gender solutions knee shown in figure 3.6.


Figure 3.6 – The Zimmer gender solutions knee; narrower contour [Zimmer, 2011]


Obesity is known to correlate with the development of osteoarthritis (OA) in human joints and is strongly influencing the need for total knee replacements. The prevalence of osteoarthritis is known to increase with advancing age, reaching a peak between 55-64 years where around 1/5 of men and 1/3 of women are thought to be obese [Changulani et al, 2008].
A Scottish study, with 858 people aged 58 and above, found that the prevalence of osteoarthritic joint pain to be twice as high in overweight people. Additionally, an independent Australian study, involving 7500 subjects, also found that overweight adults are twice as likely to be diagnosed with arthritis [Busija et al, 2007].
Obesity can be defined by the World Health Organization’s classification of body mass index (BMI). Derived from a formula of age and height, BMI provides an indication of the effects increased weight may have upon a person’s health. The following classifications are widely accepted in the medical profession and are used as reference throughout this report: overweight (BMI 25-30 kg/m2), obese (BMI 30-40 kg/m2) and morbidly obese (BMI >40 kg/m2). Ultimately, excessive body weight leads to the repetitive application of extravagant forces on the knee joint, resulting in degenerative wear of the shock absorbing meniscus. The force exerted on the knee is a function of body mass (F = Mg) Therefore, with the meniscus estimated to experience around 50-70 percent of the load applied to the knee and these forces exceeding 4x body weight in everyday tasks such as walking down stairs, it is feasible to conclude that obesity is an independent factor in the degradation of the human knee meniscus.

Figure 3.7 – The force exerted on the knee of an obese patient compared to a normal patient [Mazzara, 2010]

On the other hand, with degenerative wear also heavily affecting the human joints of obese patients that do not serve a specific load bearing function, such as the fingers, it is now recognised that the highly metabolic and inflammatory environments of body fat may influence joint degradation. Fat tissue is thought to secrete active agents including adipocytokines, such as leptin, resistin, and adiponectin, which scientists believe may influence osteoarthritis through direct joint degradation or through control of local inflammatory processes [Mazzara, 2010]. Body levels of these damaging proteins have generally been found to be higher in obese individuals, leading to increased interest in this research subject.

Obesity is a growing world epidemic, with 2005 figures estimating around 400 million of the global population to be obese, and predicted to rise to 700 million by 2015 [BBC, 2008]. In the United States currently 71 percent of individuals over the age of 60 are classified as being obese. The increased consumption of energy-dense ‘western’ foods in conjunction with poor physical activity is thought to be the cause for obesity rates increasing three-fold since 1980 in countries such as North America, Eastern Europe, the UK, Australasia and China [WHO, 2003]. By far the worst affected region in the world is the South Pacific Islands, where nations such as Nauru, Tonga and Cooks Islands, with obesity rates between 65-80 percent [The Independent, 2010], have adapted a traditional ethos of obesity being a sign of ‘beauty and wealth’. 


Obesity has been a subject of topical debate within joint replacement for a number of years, with obese patients being advised to lose an appropriate amount of weight prior to operation and, in a bid to reduce expenditure, attempts have been made to withhold funding for surgical procedures for those patients with a BMI > 30 kg/m2 [Horan, 2006]. In a population-based control-case study performed in the UK, it was estimated that 24 percent of surgical cases of knee osteoarthritis might be avoided if overweight and obese people reduced their weight by 5kg [Williams, 2009]. Nowadays it has been appreciated that obesity does not greatly influence the successful outcome of knee replacements in overweight and relatively obese patients however; problems of mobilisation and functional outcome have been encountered in patients of BMI > 40 kg/m2. In a recent study, morbidly obese TKR patients were associated with inferior clinical outcome scores, inferior five-year survivorship and a higher rate of wound complications when compared to a matched control group of non-obese patients (BMI < 30kg/m2) [Amin et al, 2006]. Although morbidly obese patients are thought to experience similar pain relief to healthier patients following TKR, they are still likely to remain more functionally impaired, with walking distance and the ability to manoeuvre stairs being very limited. One of the main concerns is the ability for patients to lose weight. Statistically more TKR patients gain weight or remain unchanged compared to those who lose weight post-operatively. The results of Doswey et al (2010), found that one in five patients gained 5 percent or more of the pre-operative weight within 12 months of TKR. In an extreme effort, orthopaedics may need to influence weight loss surgeries in conjunction with water aerobic classes to ensure the prolongation of prosthetic knee implants. The World’s orthopaedic surgeries are expected to encounter an increasing number of patients of BMI > 40 kg/m2 as the average body-weight continues to increase, encountering the associated extravagant costs in parallel.