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The main focus of research to date has been on mechanoreceptors located in the ACL and PCL. However, there is some evidence that the medial collateral ligament (MCL) and lateral collateral ligament (LCL) contain Golgi-like tendon organs and ruffini endings (Solomonow and Krogsgaard 2001, Dyhre-Poulsen and Krogsgaard, 2000) but not pacinian corpuscles. This suggests the collateral ligaments would not provide afferent information on rapid acceleration or deceleration of the joint. De Avila et al., (1989) conclude due to the scarcity of mechanoreceptors in the collateral ligaments, they may only be important in protecting injured joints, when other mechanoreceptors in the cruciate ligaments have been lost. However, there has also been some minor evidence of a MCL-sartorius/ quadriceps reflex (Kim et al., 1995), although again this has not been well documented.

The menisci are crescent shaped areas, made up of fibrocartilage situated in the condyles of the femur and tibia (Marks et al., 2007). The majority (up to 90%) of menisci is Type 1 collagen. However, mechanoreceptors including ruffini endings, pacinian corpuscles and Golgi-like tendon organs have been identified in the medial meniscus, particularly in the posterior horn (Friden et al., 2001). There is also some evidence to suggest the medial meniscus is connected to the cerebral cortex (Pitman et al., 1992).

Dye et al., (1998) used conscious mapping of the internal structures of the knee joint without use of anaesthesia to isolate specific sensory contributions of intra-articular structures. Results indicated a level of sensory control for the majority of intra-articular aspects of the knee joint. Therefore knee joint tissues other than the cruciate ligaments may contribute some afferent information during knee movement.

Despite a wealth of research there are limitations to histological mechanoreceptor studies; the use of gold and silver chloride stains is not always accurate and vascular structures can be mistaken for mechanoreceptors (Johansson et al., 2000, McCloskey, 1978). The classification of mechanoreceptors is also inconsistent and the identification alone of mechanoreceptors does not imply functionality (Johansson et al., 2000, McCloskey, 1978). However, due to the vast amount of research on the knee joint, we can somewhat confidently conclude mechanoreceptors are present in articular and periarticular areas. Two main theories are proposed to explain the management of afferent information provided by articular mechanoreceptors. The first theory discusses H-reflexes including medial collateral, lateral collateral anterior cruciate and posterior cruciate ligament – muscular reflexes (Dyhre-Poulse and Krogsgaard, 2000). This hypothesis is not a current one, Hilton in 1863 stated “muscles, indeed, appear to be told, through the medium of the nerves of the interior of the joint, that its articular structures are overtasked” (p.169). Early direct studies were on feline animals using the cranial cruciate ligament a comparable ligament to the ACL (Cole et al., 1996). Results provided evidence of neural adaptations that would support a ligament-reflex response following stimulation of the ligament.

The most commonly researched is the ACL-hamstrings reflex (Krogsgaard et al., 2002). This theory states afferent information from the ligament creates excitation of the hamstring muscle fibres and hence acts as a protective mechanism (in this case to hyperextension) (Tsuda et al., 2001, Dyhre-Poulsen and Krogsgaard, 2000). The afferent signals from the ACL either activate or inhibit hamstring muscle spindles via the gamma motor neurone system (Dyhre-Poulse and Krogsgaard, 2000). This reflex has a latency of between 95-110ms (Dyhre-Poulse and Krogsgaard, 2000, Krogsgaard et al., 2002) and hence cannot be a protective mechanism. Pope

et al., (1979) also discredited a protective reflex theory using basic calculations of time latencies following a theoretical ski injury. They concluded the ligamento-muscular reflex would take 89ms to activate whereas the ligament would fail at 34ms following knee loads. Therefore, the authors proposed an alternative function for knee joint afferent signals during joint stability.

A more feasible theory is the contribution to pre-programming of muscle stiffness around the knee joint (Johansson et al., 1991a, Johansson et al., 1991b). It is thought joint afferent signals feed in to gamma motor neurones in the muscle spindle. This information contributes to the “final common input”, that is information from muscle spindles, joint mechanoreceptors and potentially other mechanoreceptors to regulate the stiffness of muscle and prepare the joint for

impending loads using the feedforward mechanism (da Fonseca et al., 2004, Johansson et al., 1991b, Dyhre-Poulse and Krogsgaard, 2000). This theory is more feasible as the joint afferents may provide continuous afferent signals and not just at extreme ranges of movement that inform preparation signals for impending loads (Ferell et al., 1987).

Although theories support a role for cruciate ligaments in proprioception, it is unclear exactly how much contribution can be accredited to knee joint receptors (Adachi et al., 2002, Johansson et al., 1991a) and surrounding muscle spindles (Riemann et al., 2002c) or indeed interaction between the two, for example in the “final common output” theory (Johansson et al., 1991b). Research is this area has been criticised for its indirect nature, for example considering sensory evoked potentials (SEPs) during unconscious palpation of tissues (Dye and Vaupel, 2000). Indeed the main body of literature comes from histological perspectives and is therefore lacking in ecological validity. However, it is evident that structures in the knee joint have the architecture to provide proprioception information during movement, and hence contribute to joint stability (Adachi et al., 2002). As such, it is likely all mechanoreceptors in and around the knee joint contributes to proprioception. The following section discusses the current protocols for the measurement of knee proprioception; hence the “net” proprioceptive ability of the knee joint, regardless of which classification of mechanoreceptors supplied the afferent information.