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Prophylaxis of medial compartment gonarthrosis in varus knee – current state of knowledge

The progression of gonarthrosis results in reduction of physical activity. One of the factors that increase the risk of osteoarthrosis may be joint overload related to the malalignment of the mechanical axis of the lower extremity. The medial compartment (MC) of the knee is particularly susceptible to overload due to the external knee adduction moment (EKAM). Varus knee malalignment contributes to increased EKAM and thus results in increased MC loading. The purpose of this study is to present methods described in current literature aimed at reducing the disproportion in the distribution of loads on articular surfaces of medial and lateral knee compartments in people with varus knee malalignment. Methods have been divided into non-surgical (gait training, physiotherapy, and orthopedic supplies such as valgus braces, lateral wedge insoles, walking poles) and surgical ones (corrective osteotomy).


Introduction
The articular surfaces of the knee joint are among the most frequent locations where degenerative changes occur. The progression of gonarthrosis and the related pain as well as knee range of movement (ROM) limitation result in significant reduction in physical activity and work capacity. One of the risk factors for the degenerative disease may be the mechanical overload of the joint due to lower extremity (LE) malalignment [1].
In normal conditions, the mechanical axis of the LE -the line connecting the center of the femoral head with the center of the ankle joint -goes through the center of the knee joint. In a standing position this results in equal distribution of loads between both compartments of the tibiofemoral joint -lateral (LC) and medial (MC) [2]. During gait this balance is disturbed, and around 70% of the load is borne by the MC due to medial orientation of the ground reaction force (GRF) vector to the knee joint center [3]. The main biomechanical indirect indicator of MC loading during gait is the external knee adduction moment (EKAM) [3]. The EKAM is a product of the GRF in the frontal plane (sum of vector components: vertical and lateral-medial GRF) and the length of the orthogonal moment arm running toward the center of the knee joint, termed the frontal plane lever arm [4] (Figure 1 A). The EKAM and GRF have two peaks (early and late) during the stance phase, whereas the lever arm length is relatively constant [4]. The first peak EKAM is typically the largest [4].
In recent years the relationship between EKAM and MC loading has been confirmed by studies on patients with instrumented knee implants installed, which enabled in vivo measurements of the load acting on tibial articular surfaces [5][6][7][8].
In varus knees the mechanical axis runs medially to the center of the knee joint. This causes the transition of the majority of the load onto the MC already during standing [2]. During the stance phase the increased EKAM results mainly from increased length of the lever arm (Figure 1 B) and indicates MC overload [4,9,10]. This results in a higher risk of onset [11,12] as well as further progression [13][14][15] of medial gonarthrosis in varus knee malalignment.
Some researchers also use EKAM impulse [16][17][18], which provides information on cumulative knee load through the stance phase [16], as indirect MC loading measure. Chang et al. confirmed that increased EKAM impulse contributes to me-dial gonarthrosis progression [15]. According to Walter et al., who found that increased external knee flexion moment (EKFM) may contribute to MC loading increase [8], EKFM has also been used as an indirect MC loading measure [16,18,19]. Since EKFM was not correlated with likelihood of medial gonarthrosis progression [15], we did not take it into account in this review.
The purpose of this study is to present methods described in current literature that are aimed at reducing EKAM and EKAM impulse and thereby the MC loading in patients who are at risk of early medial gonarthrosis related to varus knee malalignment.

Non-invasive actions that modify MC loading
Medial compartment loading may be decreased non-invasively with gait modifications or orthopedic supplies. These methods are aimed at reducing EKAM (GRF or lever arm) or EKAM impulse.

Gait training
There are some characteristics of gait that reduce MC loading. The first consists in increasing the foot progression angle (FPA), i.e. placing This mechanism shifts laterally the center of pressure (COP), and consequently the GRF vector runs more laterally, closer to the knee joint center, reducing the frontal plane lever arm [19], with no GRF increase [16]. Out-toeing reduces the second peak EKAM and EKAM impulse both in patients with varus knees and medial gonarthrosis [18,19] and in healthy persons [16]. Although out-toeing increases the first peak EKAM, it contributes to slowing down the medial gonarthrosis progression [20]. By contrast, other research indicated the gait with decreased FPA (in-toeing) to be more effective for individuals with medial gonarthrosis [21]. In-toeing shifts the COP laterally and the knee joint center medially [21]. As a result, the frontal plane lever arm is reduced, with no GRF increase, and the first peak EKAM decreases [16,18]. These observations, however, have not been supported by long-term studies yet, and Simic et al. found that in-toeing besides the positive effect also may increase the second peak EKAM [18]. Increased internal hip abduction moment during the stance phase also protects against medial gonarthrosis progression [22]. Stronger ipsilateral hip abductors help in maintaining the trunk over the loaded LE during the stance phasetherefore the GRF vector runs more laterally, closer to the knee joint center. As a result, the frontal plane lever arm is reduced [22]. Other studies focused on the influence of lateral trunk lean over the stance LE [16,17,23] support this observation. This gait modification results in shifting the center of body mass laterally; therefore the frontal plane GRF vector runs closer to the knee joint center and the lever arm is reduced [23]. Leaning the trunk laterally over the stance LE results in a first and second peak EKAM as well as EKAM impulse decrease in patients with medial gonarthrosis [17] and in healthy persons [16].
Medial thrust, which is a combination of slight knee flexion and internal hip rotation during the stance phase, reduces MC loading in persons with proper alignment [16,24]. However, there are no papers that report its influence on MC loading in varus knees.

Orthopedic equipment and supplies
Lateral wedge insoles (Figure 2) reduces the EKAM by shifting the COP laterally and thus reducing the frontal plane lever arm [25][26][27] (Figure 1 D). Lateral wedge insoles with additional support under the longitudinal arch of the foot improve EKAM reduction and foot function in healthy persons [28]. Soft neutral insoles also reduces MC loads in healthy persons by reducing the vertical component of GRF while walking [29].
On the other hand, patients with medial gonarthrosis respond differently to lateral wedge insoles. Although insoles may contribute to EKAM decrease [27,30], they do not necessarily alleviate the symptoms, slow down the medial gonarthrosis progression or improve joint functionality [31]. This may arise from already persistent structural changes in the foot joints that are meant to compensate for the varus knee malalignment [32]. Maly et al. reported no change in EKAM after using lateral wedge insoles [33].
Another medical product, valgus braces, produce constant valgus moment at the knee, which results in slight MC separation and pain relief [34]. The effectiveness of valgus braces in relieving the MC pain is comparable to the surgical treatment -high tibial osteotomy (HTO) [34]. Valgus braces are reported to be more effective in reducing EKAM than lateral wedge insoles [35]. Although valgus braces improve functionality and reduce knee pain, they may not be tolerated for a long period [36] due to e.g. skin irritation or impracticality [34,36].
Next, walking poles decreased MC loading in one person with force-measuring knee replacement installed, by transferring some of the GRF through the contralateral walking pole [24]. On the other hand, research on a group of patients with medial gonarthrosis and varus knee malalignment showed contrary results [37]. Although walking poles reduced GRF, at the same time frontal plane lever arm length increased and consequently EKAM increased [37].

Physiotherapy
There is a lack of thorough cohort studies in the field of physiotherapy effectiveness in varus knee malalignment decrease. A case study of a 16-year-old boy reported a reduction of varus knee malalignment from 7 cm to 3 cm of intercondylar distance after 23 months of therapy with the Postural Reconstruction method [38]. This therapy is based on neuromuscular facilitation/inhibition mechanisms with no direct manipulation on painful areas [38]. The effect continued after 3 years of observation [38].
Generally, manual therapy is considered mainly as an additional help in restoring proper LE alignment with high tibial osteotomy (HTO) [39][40][41][42][43] or directly before and after arthroplasty [44]. However, by acting on the soft tissue one may slow down the onset of medial gonarthrosis or reduce the symptoms. Bennel et al. showed that exercises aimed at strengthening hip adductors and abductors in a group of patients with varus knees and medial gonarthrosis improved knee functioning and reduced pain although no EKAM decrease was observed [45].
It is not clear whether strengthening the quadriceps in patients with gonarthrosis and LE axis malalignment is appropriate. Stronger quadriceps in persons with knee malalignment and decreased knee stability is related to higher risk of gonarthritis progression [46], and quadriceps strength is not correlated with EKAM regardless of the degree of varus knee malalignment [47]. On the other hand, Segal et al. found that stronger knee extensors in a group of people over 50 years old reduce symptoms related to gonarthrosis (pain, knee stiffness) [48]. What is more, a 12-week training program aimed at strengthening knee joint extensors in patients with medial gonarthrosis contributed to reduced pain, although only in those with a normal LE axis [49].

Surgical treatment
Surgical treatment of varus knee malalignment aims first of all at restoring proper distribution of loads on articular surfaces of the tibiofemoral joint by correcting the LE axis in the frontal plane. In children this effect may be achieved by asymmetric epiphysiodesis [50]. In active young adults a corrective HTO is performed ( Figure 3) [39,50]. It allows one to delay or avoid unicondylar or total knee arthroplasty [50], which is required in endstage gonarthrosis [51].
The effect of EKAM decrease after HTO is obvious. This procedure restores LE alignment from varus (Figure 1 B) to normal (Figure 1 A). There are only a few papers focused on the biomechanics of this procedure [40][41][42][43], and they show that after HTO in adults with varus knee malalignment and medial gonarthrosis there can be observed: EKAM decrease [40][41][42][43] that continues even 2 years after the surgery [40], improved knee joint stability [41], reduced cocontraction of the vastus medialis and the medial head of the gastrocnemius muscle [41], which is associated with an MC loading decrease [42], and knee ROM normalization [43].
On the other hand, there are many reports that confirm the clinical efficacy of this treatment [39,[52][53][54][55]. There are different opinions on the shape and the level of HTO, the choice of the stabilization device, the amount of correction or hypercorrection and indications for this surgery. These variables depend on the deformation magnitude and gonarthrosis severity. HTO clinical effectiveness is measured by lessened pain, improved stability and, as a result, total knee replacement postponement.
The HTO shape and level may depend not only on the LE alignment but also on the condition of knee ligaments. In order to improve the lateral knee joint stability, the osteotomy should be localized above the insertion of the tibial collateral ligament insertion to increase its tension. However, there are reports suggesting that there is no predominance of any HTO surgery [52] or osteosynthesis [53] technique. Fujisawa et al. suggested hypercorrection because of the possibility of lost cartilage regeneration after mechanical axis translation towards a less destroyed knee joint compartment [54]. Jakob and Murphy [55] defined the amount of hypercorrection for different degrees of knee joint space narrowing. However, there are also reports that contradict the possibility of cartilage regeneration after hypercorrection [56].

Synthesis of current knowledge and conclusions
On the basis of this review of the literature, it may be deduced that: in adults, proper exercise, manual therapy or orthopedic supplies may only reduce the MC loading (delay the onset of medial gonarthrosis). Realigning the LE axis in adults may be achieved by performing corrective osteotomy (medial gonarthrosis prevention). Since methods described in this article may contribute to the MC

A B
High tibial osteotomy loading decrease, slow down the progression of medial gonarthrosis and as a result delay the necessity of knee arthroplasty, physicians and physiotherapists should be aware of them when treating varus knee malalignment in adults. There are some contradictory reports on the effectiveness of various nonsurgical procedures used to reduce the MC loading in varus knee. Therefore certain exercises or orthopedic supplies should be prescribed for a particular patient with caution.