Foot posture assessment is important to consider when using LWI for the treatment of medial compartment knee OA, as foot posture variations are associated with the development of some lower limb abnormalities and musculoskeletal conditions (Reilly et al., 2009, Levinger et al.,
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2010, Abourazzak et al., 2014, Buldt et al., 2015, Buldt et al., 2015,) and altering of the mechanical alignment and dynamic function of the lower limbs (Levinger et al., 2010), therefore possibly reducing the effectiveness of various interventions such as LWI in the treatment of medial knee OA. For example, low arched (pes planus) and pronated (everted) feet have been associated with medial compartment knee OA within the literature (Levinger et al., 2010, Levinger et al., 2012, Buldt et al., 2015). However, despite this observation, the underlying mechanics linking foot posture and medial compartment knee OA are somewhat unclear. Therefore, Levinger et al., (2010) advocates an in depth knowledge of foot structure to be paramount in fully understanding the effect of interventions on the knee and lower limb joints in order to identify participants who will most likely benefit from intervention (Levinger et al., 2010). Currently however, there is a lack of research surrounding medial knee OA and foot structure and therefore greater investigation is required (Levinger et al., 2010, Levinger et al., 2012, Levinger et al., 2013).
Accurate foot assessment can provide an appreciation into how foot postures may influence or be influenced by reducing the loading on the medial knee compartment (Reilly et al., 2009, Levinger et al., 2012). Levinger et al., (2012) identified subjects with medial compartment knee OA to demonstrate altered foot kinematics during gait that are symptomatic of a less mobile, more everted foot type. Similarly, Reilly et al., (2009) compared navicular height in sitting and standing positions in 60 subjects with hip OA, 60 subjects with knee OA and in 60 controls. No difference was found between the knee OA and control groups; however, there was a considerable difference in frontal plane calcaneal angle, indicating a more everted (pronated) rearfoot in the knee OA group.
Foot pronation has been suggested to potentially reduce the adduction moment by shifting the centre of pressure laterally indicating an adaptation by the foot to reduce the load on the medial compartment of the osteoarthritic knee (Desal et al., 2007, Levinger et al., 2010, Levinger et al., 2013, Abourazzak et al., 2014).
It is well known LWI can alter foot motion (Levinger et al., 2013, Jones et al., 2015), specifically leading to an increase in rearfoot pronation (Nester et al., 2003, Kakihana et al., 2005, Abourazzak et al., 2014, Jones et al., 2015). Tibial malalignment and the extent of rearfoot range of motion identified within OA subjects was hypothesised to affect individual responses to load-altering interventions, such as LWI (Levinger et al., 2012). Accentuating pronation using a lateral wedged insole on an already pronated foot could potentially contribute
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to detrimental changes to lower limb kinematics and therefore the development of musculoskeletal disorders in other areas (Levinger et al., 2010, Abourazzak et al., 2014). However, there is currently a lack of research assessing foot posture and the effect on the EKAM and effectiveness of LWI concurrently (Levinger et al., 2012).
One of the most validated methods for assessing foot posture is the foot posture index (FPI) (Redmond et al., 2008). The FPI was developed to provide an efficient and reliable method for assessing static foot position, and is routinely used for clinical and research assessment (Redmond et al., 2008). A study compared FPI scores between 20 patients with medial knee OA and 20 controls and reported a considerably elevated average score in those with medial knee OA. This indicates a more pronated foot posture type in the medial knee OA population (Reilly et al., 2009, Levinger et al., 2010). A more recent study by Buldt et al., (2015) investigated the differences in the EKAM in healthy individuals with normal cavus (high medial longitudinal arch) or planus (low medial longitudinal arch) foot postures using the FPI. Results indicated that foot posture does not considerably influence the EKAM in healthy individuals whilst walking at a comfortable pace, suggesting the biomechanics of the knee are not substantially influenced by foot posture in healthy individuals. This finding proposes that foot posture may be altered by medial compartment knee OA. Also that the incidence of pronated feet within the medial compartment knee OA population presented within the literature may be a mechanism adapted by the individual to reduce disease symptoms (Buldt et al., 2015), instead of the presence of a pronated foot being the cause of medial compartment knee OA. Likewise, Gross et al., (2011) found that planus foot morphology is associated with medial compartment knee OA.
The use of the FPI may allow further detailed analysis of the EKAM primary outcome measure by grouping the subjects using these classifiers (Keenan et al., 2007, Redmond et al., 2008, Wrobel and Armstrong, 2008).
Levinger et al., (2012) also advocates an insight into the dynamic function of the foot during gait to be important in understanding the effect of foot kinematics on loading of the knee, thus providing an appreciation into the factors affecting the EKAM, aiding the design of knee OA treatment strategies. Several previous investigations have examined dynamic function of the foot during gait using three dimensional (3-D) motion analysis systems with infra-red cameras and force platforms in order to capture and analyse the motion of the lower limbs, ground reaction forces and also to identify gait cycle events (Landry et al., 2007, Levinger et al., 2012,
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Jones et al., 2014, Jones et al., 2012, Shultz and Jenkyn, 2012). The FPI and dynamic foot motion results can be used to gain an understanding into possible relationships between clinical and biomechanical foot measurements and their influences on the loading on the medial compartment of the knee.