Study protocol: the effect of whole body vibration on acute unilateral unstable lateral ankle sprain- a biphasic randomized controlled trial

Lateral ankle sprains are one of the most common musculoskeletal injuries[1, 2]. They mostly result from an internal rotation and adduction of the plantarflexed foot with subsequent damage to the lateral capsulo-ligamentous complex. About 65% of all lateral ankle sprains are isolated anterior tibio-fibular ligament (ATFL) injuries, while 20% are combined ATFL and calcano-fibular ligament injuries[3]. 85% of all ankle injuries are ankle sprains and 85% of those are inversion sprains[4], with about one ankle sprain occurring per 10.000 people every day[5]. Although usually considered an innocuous injury, three-year full recovery rates range from 36% to 85%[6].

Ankle instability is most likely caused by damage to passive structures and neuromuscular impairment[7]. Damage to passive structures, i.e. capsular structure and ligaments, results in objective (anterior drawer, talar tilt) and subjective instability (giving-way)[3]. Neurological impairments include muscle fatigue[7‐9], reduced dynamic balance[10‐12] and impaired postural control[13‐15]. Although not all patients with functional (FAI) or chronic ankle instability (CAI) present with objective instability, they are all thought to have neuromuscular impairments[2, 16, 17]. This results in lower physical activity levels[18], diminished quality of life[19], and a possible increase in the risk of osteoarthritis[20, 21].

Any treatment plan should therefore address both damaged passive structures and neuromuscular impairment. Generally, three treatment regimens are available: functional treatment (early mobilization and bracing), cast immobilization for 2–6 weeks, or operative treatment[22, 23]. A Cochrane review from 2007 comparing conservative and surgical treatments found similar outcomes[3]. Therefore, functional treatment has to be considered the current gold standard. Functional treatment should allow healing of damaged passive structures and compensate for neuromuscular impairments, i.e. improve muscle strength and proprioception. Physical therapy therefore includes stretching, body-weight exercises, training apparatus and devices (for example wobble board training). Supervised rehabilitation has proven superior to conventional treatment[24, 25].

A neuromuscular training method gaining increased attention is whole body vibration (WBV). WBV involves synchronous or side-alternating sinusoidal vibrations, which are transmitted to the body via platforms. It is believed to evoke muscle contractions via stretch reflexes in the muscle spindle system[26‐28]. A growing body of evidence indicates improvements of various neuromuscular parameters following WBV, such as power, strength, movement velocity, range of motion and balance[29‐38]. Reports to the contrary[39, 40] may be due to heterogeneous patient populations and varying intervention protocols, which in turn can be due to limited knowledge of the dose-effect relationship, i.e. vibration frequency and amplitude.

In general, frequencies <10Hz are believed to loosen muscle and tissue. Frequencies >10Hz and <20Hz still allow active contraction and relaxation of muscle fibers and are used for coordination exercises. Frequencies >20Hz result in isotonic muscle contraction[41]. Vibration amplitude seems to be positively correlated to muscle activity[42‐44]. Although WBV has numerous contraindications, as listed in Table 1, the only adverse side effects reported are temporary hyposensitivity of the foot soles[45].

With WBV improving range of motion (ROM), power and balance - parameters known to be affected in patients with ankle instability - it might be beneficial to include it into the current functional treatment regime. To our knowledge only one study investigated the effects of WBV in patients with ankle instability. Cloak at al.[49] conducted a randomized controlled trial (RCT) on functional ankle instability in dancers (n=38), comparing WBV to a control group (regular dance training). The WBV group yielded significantly better results for balance (STAR extrusion balance test) as well as a significantly reduced Center of Pressure values.

The present study protocol on the effect of WBV in patients with acute, unilateral, unstable inversion ankle sprains is the first study to apply WBV in patients with acute soft tissue injuries. Due to its influence on various neuromuscular parameters, which are known to be impaired in patients with ankle instability, WBV provides a novel, functional treatment approach for this problem.

Several limitations of the present protocol must be discussed. First of all, the authors have decided to use a functional classification system. With ankle instability being the most important parameter for further treatment decision, an established functional classification system was chosen. Ankles are classified into stable or unstable based on clinical presentation and examination (swelling, anterior drawer test and talar tilt test)[46‐48]. In case initial examination is not tolerated, delayed physical examination has been shown to be equal to arthrography (specificity/sensitivity: 85%/96%)[63‐65]. Its feasibility in daily practice and high sensitivity makes it a suitable classification system for patients with acute inversion ankle sprains.

Second, as discussed in the introduction, functional treatment seems to be the current gold standard treatment approach. Still, immobilization, as well as type and duration of physiotherapy are a matter of discussion. Kerkhoffs et al.[66] conducted a systematic review on the effectiveness of various braces/bracing methods for acute ankle sprains and found lace-up supports to be the most effective, which seems supported by a Cochrane Review[3]. A recent single-blinded RCT by Lamb et al.[67] found a short period of immobilization in an Aircast brace to result in faster recovery than double-layered tubular compression bandage. This is in line with a review by Kemle et al.[23] who found evidence pointing towards the superiority of ankle braces. Consequently, initial immobilization will be realized with an ankle brace. Moreover, the type and duration of functional rehabilitation remains unclear. Our supervised rehabilitation protocol is based on the guidelines of the Royal Dutch Society of Physical Therapy[52], which is comparable to other published treatment protocols[53‐57, 68]. Although the optimal intensity of physical rehabilitation remains unclear, 45 minutes, twice to three times a week over a period of 6–8 weeks seem to be beneficial[10, 57‐59, 69]. Due to administrative issues, we decided on supervised rehabilitation for 30 minutes, twice weekly for 6 weeks, with additional home exercises.

A third possible limitation are the chosen outcome parameters. Whereas power, assessed by a single leg vertical jump, has been proven significantly different between patients with recurrent ankle sprains and healthy controls[14, 70‐72], there is no consensus in literature as to which method best assesses balance. A commonly used method is center of pressure (COP)[73‐76]. Whereas COP assessed during single leg stance with eyes open was ineffective in identifying patients with ankle instability[73, 74], this could be achieved with single leg stance with eyes closed[75, 76]. Furthermore, COP provides an objectively verifiable parameter. Other commonly used outcome parameters such as return to work or return to sports are parameters not necessarily coinciding with the recovery of the injured ankle. They highly depend on the patient’s attitude attitude, schedules and type of job/sport.