Abstract
[Purpose] The purpose of this review was to summarize available methods and equipment which are administered to evaluate the balance in anterior cruciate ligament reconstruction. [Subjects and Methods] A literature search was performed and reviewed using the narrative approach. This study reviewed English articles concerning balance assessment methods in anterior cruciate ligament reconstruction subjects from 1985 to 2015 using the following key words: Anterior Cruciate Ligament Injury, Anterior Cruciate Ligament Reconstruction, Postural Control, Equilibrium, Balance and Stability. [Results] This review included 44 studies out of 117 initially retrieved articles. These articles were discussed in balance measurement procedure without comparing the effect of various surgical, medical or rehabilitation approaches. [Conclusion] Biodex and EquiTest, functional dynamic tests are of choice in the evaluation and tracking of anterior cruciate ligament reconstruction subjects. Force plate parameters provide information regarding strategies of static postural control and is not sensitive enough to challenge postural control system in physical activities.
Keywords: Anterior cruciate ligament injury, Anterior cruciate ligament reconstruction, Postural control
INTRODUCTION
The anterior cruciate ligament (ACL) of the knee is the principal factor controlling knee forward movement1) whereas it is the most commonly injured ligament of the knee2). Seventy percent of these injuries are non-contact ones which occurred as a result of rotation, sudden change in direction, jumping and falling3).
ACL injury is a fundamental problem among active and mature youths which in average causes 11,300 outpatient and 7,500 ACL reconstruction surgeries. ACL is rich in mechanical receptors and is known as somatosensory organ in postural control4). Thus, ACL injury causes joint instability and giving away episodes during the activities, and may be followed by limitation in physical and sport activity and low quality of life5, 6). Postural control deficiency is reported following ACLR7). Biomechanical changes8), changes in quadriceps muscles7, 9) and brain activities and proprioception disorders9) have been reported following ACLR. As the main criterion for the ACL reconstruction (ACLR) surgery, the individual’s ability to return to the activities of daily living (ADL) and to the recreational and sports routines are the main concerns for those who suffer from ACL injury and is being considered10).
Various studies have investigated the mechanism of equilibrium deficiency following ACLR7, 8, 11); some showing an increase in postural sway and some showing no change9) although no comprehensive study to date has investigated the available methods for evaluating equilibrium after ACLR. With regard to the present body of literature on the equilibrium after ACLR, the aim of the present study is to review all the methods applicable in studying equilibrium following ACLR.
SUBJECTS AND METHODS
We searched MEDLINE and allied health literature for studies from 1985 through 2015 by using the following Medical Subject Heading (MeSH) terms and keywords: Anterior Cruciate Ligament Injury, Anterior Cruciate Ligament Reconstruction, Postural Control, Equilibrium, Balance and Stability. All original and review articles were included when they were written in English and when they were addressed using balance assessment methods in ACL injury and ACLR patient. The article may be included only if they were studying the variation in ACLR subjects without including medical or physical management interventions. Thus, the studies about kinds of ACL surgery, those comparing various rehabilitation or medical management strategies, and the studies of the effect of physical training procedures were not included in this review. Instead, case control studies, cohort studies and evaluation records were included. References of included studies were checked for additional studies meeting the inclusion criteria. A total of 44 studies were found with relevant data on our aim. These studies were categorized according to our kinds of balance assessment methods and reviewed in a narrative manner.
RESULTS
The primary search with keywords provided 106 articles. Moreover, 11 articles were found from communication with content experts in the field of ACL screening and rehabilitation in the 2010–2015 conference proceedings of the American College of Sports Medicine and European Society of Sports Traumatology, Knee Surgery, and Arthroscopy. From these 117 articles, 70 articles were excluded for not meeting the inclusion criteria, and 27 articles were excluded after comprehensive review of their method section. Finally, 20 full text articles were included in the present study.
DISCUSSION
Balance and postural control: Balance and postural control is a basic component of all motor skills. It is a complex function and a cognitive process12) that requires integrated coordination of sensory inputs from vestibular, visual and somatosensory systems. Insufficiency of any of these system results in balance impairment and postural control problems5).
Mechanisms of BPC: Neural components of BPC are subdivided into: sensory, motor and cognitive sections. Cognitive part moderates the function of sensory and motor sections function response to imposed tasks or environmental changes and prepares them to deal with the changes2).
Methods to evaluate the balance: Different studies have used various methods to assess BPC. Objective and variables of a study are the key criteria to select BPC measurement methods and interpret its findings.
Force plate (FP): Static postural control is the ability to maintain position in single and double limb stance and is presented by measuring displacement, velocity, acceleration and frequency of displacement of center of pressure (COP). In quiet standing posture, the body is fairly immobile and COP has a form of rocking motions named sway. Force plate records COP location in space at any time and help the assessor to evaluate subject’s ability in maintaining COM inside BOS13). The force plate data is used to calculate variability and regulation of position, in order to determine instability in maintaining static posture14). However, force plate is not sensitive enough to challenge postural control system in physical activities15).
Biodex stability system (BSS): BSS consists of a multi-axial platform. This system identifies subject’s ability to maintain dynamic stability on an unstable inclined platform. With selecting instability level, the subjects try to maintain the platform. Instability level is adjustable between the most stability level, level eight and the least instability level, level two. BBS produces three indices electronically which are named based on the degree of AP and mediolateral (ML) slop. The indices are anteroposterior stability index (APSI), mediolateral stability index (MLSI) and overall stability index (OSI). These mechanical tests could not challenge falls due to external perturbations during physical activity16).
Equitest system: Equitest system is used to produce perturbations beyond BOS and measure balance reactions. It contains a pair of force plates (dual force plate) and produces perturbation of linear displacement (translatory) type. The system sets the displacement of support surface according to subject’s height in a way that all the subjects get disturbed to the same angle (in degree) and with the angular moment in a defined period of time. The device screens data from sway (in degrees), shear force and COP and COM displacements in X and Y axes17).
The role of ACL in balance and postural stability: Recent studies have provided strong evidences regarding sensory role of ACL8). It has even been proposed that proprioceptive neuro-physiological function of ACL is as important as its biomechanical function in the maintenance of joint stability10). If afferent information changes following ACL injuries, alteration in processing of cortical information is also expected. These changes have been confirmed by the study of evoked potentials and electroencephalography (EEG)5).
ACL injury and its consequences in balance control process: Postural control is not normal following ACLD and ACLR7). Numerous studies reported proprioception impairment after ACL injury18, 19). Impairment of afferent or efferent systems may damage balance control system; having this fact in mind that cognitive system is an-important component of the balance control system. Previous studies approved increased sway amplitude and frequency in ACLD1, 7) and ACLR7, 8) subjects compared to healthy individuals. These differences may be related to impairment in central mechanisms of postural control4, 5, 7). Consequently, despite the fact that ACL injury is basically a peripheral musculoskeletal problem; it is currently mulled over a neurological dysfunction rather than a simple neuromuscular deficit20).
In some studies, single limb stance (SLS) tests are used to measure subject’s postural control21). In ACL individuals, the sway is the same as standing on either sound or involve limb and is higher than that of healthy subjects. That means, ACLD affects both limbs irrespective of the injury side. This finding supports the central impairment theory7). That means peripheral injuries may involve central data processing system even without structural damage to processing pathways22).
Balance assessment methods in ACLR: A) Force plate (FP): Single limb balance seems not to be significantly different between sound and affected limb of 10–18 months post-ACL23). However, it has been demonstrated that ACLR subjects have better balance standing on affected limb when compared to chronic ACLD subjects although the balance in both groups was significantly defective when compared to healthy individuals24, 25).
ACLR balance on sound and affected limb is controversial. In 1999, Haffman et al.9) evaluated static and dynamic balance in single limb support on FP in twenty ACLR individuals. They used electrical stimulation on tibia nerve to perturb subjects and measure their dynamic balance performance. The difference between ACLR and the control healthy group was statistically significant only in dynamic context. The required time to retrieve equilibrium following perturbation was significantly longer in ACLR group for both sound and reconstructed limbs. This finding implies that the affected side is neurologically in a mutual connection with sound side. This phenomenon accentuates the central mechanism of postural control and CNS plasticity to meet the functional needs of the subject9).
This way, the observed differences between loading rate of sound and affected limbs and between ACLR and normal subjects, proposed by Deker et al.21) in 2002, is rational.
In 2012, Mohammadi et al.13) indicated that the sway was greater in single limb standing on reconstructed limb than that of sound limb and both were more than healthy controls.
Dingenen et al., evaluated postural stability during the transition from double-leg stance (DLS) to single-leg stance (SLS) in 20 ACLR and 20 healthy subjects. ACLR group showed postural stability deficits, indicating that these persons may have a decreased ability to stabilize their body after the internal postural perturbation created by the transition from DLS to SLS26).
These researches all confirm that balance and balance control is impaired in ACLR subjects. In brief, balance and postural control impairment is one of the common consequences of ACLD and in some cases, it may not be completely resolved even months after reconstructive surgery.
B) Biodex indexes: Biodex indexes represent the accuracy of positional sense receptors in detecting BOS obliquity. They may also reflect the neuromuscular coordination to reach a dynamic stability despite standing on an unbalanced surface27). Measuring APSI, MLSI and OSI confirmed that. ACLR may help in maintaining proprioception and functional stability of the knee joint. Although postural balance seems to be significantly lower in sedentary people and either limbs of ACLR clients and postural balance on affected limb and displacement of center of gravity (COG) may be less than that of sound limb in ACLR subjects28).
C) Equitest system: Lysholm et al. studied 22 ACLD individuals using sensory organization and movement control test protocol in sagittal plane in Equitest. ACLDs show greater sway when standing on affected side on a stable surface with open eyes. Their sway was also higher than that of the control group in double standing with open eyes. Testing with closed eyes showed that ACLDs and normal subjects were not statistically different in double standing. Reaction time in double standing was longer in ACLDs with anteroposterior perturbations (motion control test) in comparison to normal group. In the same way, reaction time and sway rate in ACLD group were more than in healthy subjects in response to anteroposterior perturbation when standing on affected side. Balance impairment was documented in ACLD individuals when they were studied standing on either affected or sound side. The unexpected finding in the sound side has been explained by signals form static gamma motor neurons in a reflex activity in sound side1).
Another study in 2001 showed that ACLRs have more sway in sensory organization test with closed eyes when they were tested in double stance on an unstable BOS, while they were not different from the control group in double standing on stable BOS. ACLRs single limb standing on stable BOS showed no difference between limbs. In movement control test, reaction time for anteroposterior perturbation was longer in ACLR individuals without any significant difference between the two limbs. Instead, control group had longer latency than ACLR group. Reaction time for lateral perturbation was not different between groups. This parameter was predictably longer than medial reaction time for both groups but no significant differences were recorded between the two groups and between the two limbs in each group. Medial fluctuating rates were more than lateral fluctuation without significant differences between inter and intra-group (between limbs)8).
Postural control is the basic needs of ADL. Balance dysfunction due to musculoskeletal injuries changes performance. ACLD and ACLR are very common in youth and impose considerable financial and social burden. Balance dysfunction assessment is critical in these people for planning a comprehensive rehabilitation protocol in early stage. ACL injury seems not to impose impaired postural control in static position because this position does not challenge the balance. Conversely, in real situation, several factors and perturbation challenge the balance system, therefore; dynamic assessments may better identify balance impairments. Accordingly, Biodex and Equitest, functional dynamic tests are of choice in the evaluation and tracking of these people.
A limitation of this review is its narrative approach. The included studies were not analyzed systematically and meta-analysis of the included articles was performed; therefore definitive conclusions cannot be drawn. A systematic review on present body of literature will help to provide a more clear benefit of available strategies.
Conflict of interest
None.
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