Anterior knee pain (AKP) is a common condition affecting the knee joints of both young, physically active and sedentary individuals (Crossley et al. 2016). Anterior knee pain accounts for 25% – 40% of all knee problems presenting at sports medicine clinics; one in four of an active population is affected, leading to chronic knee pain among young adults (Coppack, Etherington & Wills 2011; Crossley et al. 2016; Dutton, Khadavi & Fredericson 2016). Anterior knee pain has a higher prevalence amongst women, with an incidence two to three times more than that of men (Dutton et al. 2016; Prins & Van Der Wurff 2009).

Anterior knee pain is characterised by anterior, peri-patellar or retro-patellar pain with an insidious onset and is exacerbated under conditions of increased patellofemoral joint (PFJ) stress (Dutton et al. 2016; Nunes et al. 2013). Aggravating activities include climbing or descending from stairs, prolonged sitting, squatting and running (Nunes et al. 2013). The aetiology of AKP is unclear with a possible multi-factorial nature and develops secondary to functional or structural mal-alignment of the PFJ (Green et al. 2014). Management of AKP remains challenging with 91% of patients with AKP reporting persistent symptoms after extended follow-up and medical management (Dutton et al. 2016). It is, therefore, important to understand the aetiological pathways that may cause the pain.

Altered proprioception has been documented in patients with AKP (Akseki et al. 2008; Baker et al. 2002; Cyrillo & Cabral 2014; Guney et al. 2015). Proprioception is ‘the use of joint position sense (JPS) and joint motion sense to respond to stresses placed upon the body by alteration of posture and movement’ (Norris 2011).

Clinical symptoms of altered proprioception may include disturbed balance and clumsiness because of disturbed motor function and joint reflex stabilisation. It has been proposed that the altered proprioception amongst individuals with AKP can result in recurrent and persistent pain, and that this could increase the risk of joint osteoarthritis in the long term (Clark, Röijezon & Treleaven 2015; Röijezon, Clark & Treleaven 2015). Altered proprioception in individuals with AKP could be as a result of mechano-receptor damage associated with abnormal movement patterns and motor reactions (Clark et al. 2015; Han et al. 2016; Hillier, Immink & Thewlis 2015).

The altered movement patterns and motor reactions may be underpinned by weak quadriceps muscles, altered timing of the vastus medialis oblique muscle and altered tissue flexibility of the quadriceps and hamstring muscles (Kaya et al. 2010; Lankhorst, Bierma-Zeinstra & Van Middelkoop 2012). Altered motor reaction in AKP may be because of altered proprioceptive input from muscle spindles in skeletal muscles (Röijezon et al. 2015). Muscle spindles in skeletal muscles are a type of mechano-receptor and a major source of proprioceptive feedback (Röijezon et al. 2015). Another possible reason for altered proprioception in a population with AKP could be small nerve damage in the lateral retinaculum of the patella (Sanchis-Alfonso & Rosello-Sastre 2003). This nerve damage in the lateral retinaculum may be because of the mal-tracking of the PFJ (Sanchis-Alfonso & Rosello-Sastre 2003). It is unclear whether altered proprioception could be a risk factor leading to the onset of AKP or a resulting factor contributing to the chronicity of the condition.

The ability to accurately sense joint position is essential to aid an individual’s response when the body encounters stress (Smith, Davies & Hing 2013). Joint position sense testing can be conducted under active (biasing joint mechanoreceptors) or passive (stimulating joint and muscle tendon mechanoreceptors) conditions (Roijezon et al. 2015). Joint position sense is the ability of an individual to accurately reproduce a target angle (TA) (Baker et al. 2002; Selfe et al. 2006). The most common clinical metric used to assess JPS is absolute error (AE). Absolute error refers to the difference between the TAs and the response angle (Han et al. 2016; Hillier et al. 2015; Ogard 2011; Röijezon et al. 2015).

There is limited research investigating JPS in individuals with AKP (Yosmaoglu et al. 2013). Previous studies in this field reported no significant differences in JPS between individuals with and without AKP (Bennell et al. 2005; Naseri & Pourkazemi 2012; Yosmaoglu et al. 2013). In contrast, other studies have observed that JPS is significantly impaired in individuals with AKP compared with controls (Akseki et al. 2008; Baker et al. 2002; Cyrillo & Cabral 2014; Guney et al. 2015). The reasons for these conflicting findings are unknown and although all these studies measured JPS, different methods for measuring JPS were used. These methods included: (1) image recorded where photography is used to assess knee joint angles (Baker et al. 2002; Naseri & Pourkazemi 2012), (2) electro-goniometry with knee angular error (Akseki et al. 2008; Cyrillo & Cabral 2014), (3) dynamometry (Guney et al. 2015) and (4) a functional loaded squat system (Yosmaoglu et al. 2013).

Three studies measured non-weightbearing (NWB) JPS only (Akseki et al. 2008; Guney et al. 2015; Yosmaoglu et al. 2013), whereas the other studies combine weightbearing (WB) and NWB. Weightbearing positions have been shown to be more accurate; however, NWB knee JPS has the greatest potential for isolating the proprioceptive status of the knee joint only (Stillman & McMeeken 2001). There were also differences in the included study samples. The studies that found no significant differences used very specific populations that included pain-free participants with induced AKP (Bennell et al. 2005), athletes (Naseri & Pourkazemi 2012) and women (Yosmaoglu et al. 2013). In the studies with significant differences, activity levels were not monitored.

However, in these studies (Akseki et al. 2008; Baker et al. 2002; Cyrillo & Cabral 2014; Guney et al. 2015) participants presented with a mean duration of symptoms ranging from 3 to 36 months, indicating sub-acute ongoing chronic pain. Two of the studies focused on females only (Cyrillo & Cabral 2014; Guney et al. 2015) and one study (Guney et al. 2015) required participants to present with at least moderate pain levels (more than 4/10) that were present daily for at least a month.

The variation in methodology could also be because of the lack of a gold standard test to assess JPS. Furthermore, the tests used in these studies have poor clinical applicability and are poorly evaluated and reported on by the respective authors (Guney et al. 2015). Current studies use 1D and 2D systems only. It is possible that these methods do not have adequate sensitivity to detect small changes in the AE. The VICON 3D movement analysis system is considered the gold standard objective measure for 3D postural analysis (Brink et al. 2013).

An additional problem with the current evidence is the lack of a clear classification system of what constitutes ‘good’ or ‘poor’ JPS. The current evidence on participants with AKP only compares differences between AKP and control groups with no clear interpretation of what the values mean. Relph and Herrington (2016) assessed 116 healthy, pain-free participants between the ages of 18 and 82 years. They assessed the normative JPS AE values into flexion and extension. The mean AE values ranged between 3.1 and 3.9 degrees for flexion and 2.5 and 3.9 degrees for extension. The minimal detectable difference for JPS has been reported to range from 1.23 to 2.14 degrees in AE scores (Relph & Herrington 2015). Clark et al. (2016) reported that AE scores range between 3.18 and 5.97 degrees in healthy pain-free adults. In healthy participants, Callaghan et al. (2002) regarded an error of less than five degrees as good proprioception and an error of more than five degrees as poor proprioception.

It is unknown how much of a difference in AE is required to result in an increased injury risk. However, based on the recommendations from these studies (Callaghan et al. 2002; Clark et al. 2016; Relph & Herrington 2015) we used an AE greater than five degrees as an indication of poor JPS for the purpose of our study.

More research is needed to establish if individuals with AKP present with altered knee JPS as this could potentially lead to altered movement patterns and potentially chronic symptoms. Clinicians need to know whether to address JPS in rehabilitation when treating patients with AKP. The aim of our study, therefore, was to determine if JPS is altered in the knees of individuals with AKP. Our study is the first to use the Vicon 3D motion analysis system to accurately measure knee JPS during WB in an AKP population.

A cross-sectional, descriptive study was used to collect data. All included participants presented with AKP, with either unilateral or bilateral AKP. Two groups were formed and the knees with and without AKP were compared. We conducted our study at the Human Movement Analysis Unit, Faculty of Medicine and Health Sciences, Stellenbosch University.

Sample recruitment was aimed to attract individuals with AKP from different socio-economic backgrounds, sporting codes and areas. Letters of invitation were sent to various universities, sports clinics, physiotherapy practices and sporting clubs. The advertisements requested for volunteers who were between the ages of 14 and 40, who had pain at the front of the knee during common aggravating activities such as squatting and stair climbing and no previous lower limb surgery. Potential participants emailed the research team and completed the screening questionnaire. Participants who complied with the AKP screening tool and the diagnostic checklist (Leibbrandt & Louw 2017), which was based on a systematic review of diagnostic studies and developed specifically for our study, were considered for inclusion.

Screening was undertaken by the first author (CR) who had been trained to use the screening tool by the author with experience in treating AKP (DL). Participants with unilateral and bilateral AKP were considered for inclusion. In cases where both knees were affected with AKP, both knees were tested for altered JPS and compared with knees without AKP.

Joint position sense in people with AKP remains a controversial topic. Our study which included a young cohort of South Africans showed that there were no significant differences in knee JPS when comparing the knees with AKP to those without AKP during SLS or the AKE positions and adds to the knowledge base of JPS in young people with AKP. However, a subgroup of participants with AKP and impaired JPS bilaterally was identified. Approximately a third of the study participants were in this subgroup. In addition, there was a tendency towards a larger knee flexion in the TA of knees without AKP. This could be because increased knee flexion has been linked to increased pain in individuals with AKP because of increased PFJ contact stress (Besier et al. 2005).

Research into AKP and JPS has yielded inconsistent findings. Some studies concur with our findings of no significant difference in the perception of JPS in knees with and without AKP (Bennell et al. 2005; Naseri & Pourkazemi 2012; Yosmaoglu et al. 2013). These studies also used JPS to evaluate knee proprioception and calculated the AE to express JPS. On the contrary, others have found that JPS is more affected in people with AKP compared with individuals without AKP (Akseki et al. 2008; Baker et al. 2002; Cyrillo & Cabral 2014; Guney et al. 2015). These contradictory findings could be a result of differences in study methodology and participant characteristics.

Some of the unaffected knees in participants with AKP also had altered JPS according to the classification criteria (Callaghan et al. 2002). Similar results have been reported when comparing JPS in individuals with AKP compared with healthy controls (Baker et al. 2002; Cyrillo & Cabral 2014). These authors found the perception of JPS to be altered in those with AKP and in healthy controls without AKP. The interpretation of altered JPS in unaffected knees of participants with unilateral AKP is unclear but could be because of compensatory mechanisms during gait in people with AKP (Barton et al. 2009). In an attempt to decrease PFJ reaction forces and to avoid pain, individuals with AKP may develop a quadriceps avoidance gait pattern (Sanchis-Alfonso et al. 2016).

We defined abnormal knee JPS as an AE equal or greater than five degrees. This criterion was based on a study by Callaghan et al. (2002), who tested the criterion on healthy participants. It must be considered that classification of abnormal JPS has not been used in a population affected with AKP, making the interpretation of the findings difficult in this population (Callaghan 2011). The group of knees without AKP was smaller than the knees with AKP making it very difficult to compare the proprioceptive findings. Inter-subject comparisons for each participant’s proprioceptive outcomes were not compared and should be considered in future studies.

The observation that a small percentage of the knees with AKP and the knees without AKP had an AE greater than five degrees cannot be generalised to the rest of the population because of the small sample group.

We included participants with unilateral and bilateral AKP, which differs from other studies that used control groups without AKP for comparison. Anterior knee pain symptoms in our study were not restricted to just one knee, as more than half the individuals reported both knees being symptomatic, as AKP can be present in both knees (Kurt et al. 2016). If the pathological and normal knees are affected this could indicate that JPS should be considered in the aetiology in this subgroup (Akseki et al. 2008). If this is the case, addressing JPS in the early phases of rehabilitation might be useful to prevent chronicity of symptoms.

Studies with similar findings have included an athletic population (Naseri & Pourkazemi 2012), female athletes diagnosed with AKP (Yosmaoglu et al. 2013) and healthy participants with induced AKP (Bennell et al. 2005). Compared with pain-free controls, AKP is unlikely to affect the knee JPS in athletes (Naseri & Pourkazemi 2012). This could depend on the severity of knee pathology, pain intensity and level of physical activity amongst the athletes. Furthermore, it is unclear whether their proprioceptive abilities were significantly affected or not by the severity of knee pathology and pain intensity and whether or not their proprioceptive (JPS) ability was improved by functional state. Athletes with moderate pain may not present with proprioceptive deficits, as higher levels of motor function could account for increased proprioceptive feedback from adjacent joints and muscles (Naseri & Pourkazemi 2012; Smith et al. 2013). Therefore, it is possible that only more sedentary participants with AKP present with altered JPS.

Our participants were included irrespective of their participation in sport or activity levels, whereas other studies include athletes or restrict the population to only women diagnosed with AKP. The literature highlights that AKP is common amongst active individuals and has a higher incidence in female athletes (Neal et al. 2016; Prins & Van Der Wurff 2009). Our population is reflective of these findings, with females constituting the majority of our study population: two-thirds were active individuals and 44% where runners.

The severity of the AKP pathology, participants’ pain levels and activity levels are thought to have influenced proprioceptive function in these studies. The pain levels of our participants ranged between zero to nine out of 10 with a mean pain level of 4.5/10. It can be argued that in order to influence proprioceptive abilities, pain severity levels need to be higher than moderate (Naseri & Pourkazemi 2012).

Joint position sense was tested in both a WB (SLS) and NWB position (AKE). Other studies predominantly tested JPS in only a NWB position (Akseki et al. 2008; Bennell et al. 2005; Cyrillo & Cabral 2014).

Weightbearing test positions are more likely to be relevant in an AKP population as this is when patients typically experience pain. However, JPS in WB positions need to be measured accurately and reliably. In more recent studies, the Biodex 360 has been used to measure knee JPS. The Biodex 360 is considered a reliable measurement tool to evaluate knee JPS, however it can only account for a NWB test position.

Vicon 3D motion analyses allowed for testing knee JPS during both standing and sitting. The Vicon 3D motion analysis system is regarded as the gold standard in motion analyses (Ehara et al. 1997; Richards 1999). This is the first study to make use of the Vicon 3D motion analyses to assess knee JPS in an AKP population, which allows for better measurement accuracy and a WB assessment of JPS. Joint position sense testing in a WB position has been found to be more reliable and accurate for JPS compared with NWB test procedures (Stillman & McKeeken 2001). However, NWB testing had a greater potential to isolate the proprioceptive status of the knee joint only. Therefore, it is recommended to use a combination of both positions.

We investigated JPS in individuals with AKP and found that JPS is not significantly more impaired in knees with AKP compared with knees without AKP. The use of the Vicon 3D motion analysis system added to the measurement precision. Our findings suggest that a subgroup of individuals with altered JPS may exist in an AKP population, however, longitudinal studies are needed to establish how altered JPS relates to the pain experienced.