Dynesys: The current evidence

The Dynesys spinal system is a non-fusion pedicle screw-based stabilisation system introduced by Dubois in 1994. It consists of conventional pedicle screws with spherical head holes through which polyethylene terephthalate (PET) string is passed. A polyurethane sleeve is then passed over the string and fitted between the two pedicle screws with 300N tension. Dynesys for the treatment of discogenic back pain is thought to work by offloading the degenerate disc. The spacer leads to a loss of segment lordosis. Once the spinal extensor muscles restore lordosis, the disc space is distracted, thus offloading the affected segment.

Since its introduction, a number of studies looking at the biomechanics and clinical outcomes of patients with Dynesys have been published. This article provides a mini-review of the Dynesys literature to date.

Biomechanical Studies
An early cadaveric study by Schmoelz et al.(2003) investigated the magnitude of stabilisation on the bridged and adjacent spinal segment. They concluded that Dynesys restored motion to levels similar to intact spines, providing substantial stability in case of degenerative spinal pathologies and can therefore be considered as an alternative method to fusion surgery in these indications while the motion segment is preserved. An in vivo MRI study by Beastall et al. (2007) showed that the Dynesys Stabilization System allows movement at the instrumented level, albeit reduced, with no significant increased mobility at the adjacent segments.

However, later studies show that the restriction of movement was greater than previously thought. Several studies demonstrated a significant reduction in flexion, extension and lateral bending (Meyers 2008, Niosi 2008, Schulte 2008, Gédet 2009).

Schulte et al. (2008) described a restriction of flexion, extension and lateral flexion by 68% compared to a normal spine, with only a 13% reduction in axial rotation. Gédet et al. (2009) confirmed similar findings and concluded that the Dynesys system does not fulfil the role of a flexible posterior implant, especially as it is placed far removed from the natural centre of rotation of the intervertebral joint. More recently, cadaveric studies comparing Dynesys to rigid fixation found no difference in range of flexion and extension movement. Once again, there was little restriction of axial rotation (Delank 2010, Schilling 2010).

These last two studies question the purported advantages of flexible stabilisation which was the theoretical reduction in adjacent segment wear. Delank et al. (2010)noted less motion in adjacent segments when comparing Dynesys to rigid fixation. In comparing Dynesys to a native spine however, Liu et al. (2010) showed greater motion at adjacent levels compared to the normal spine, in addition to greater annulus stress and facet loading. The spacer length has also been shown to have an important effect on segmental range of movement and kinematics, which could affect clinical outcomes (Noisi 2008)

Clinical Outcomes
The original indications for Dynesys included segmental hyper/hypomobility, discopathy with or without functional instability and single or multiple level spinal canal stenosis with post-operative instability (Bothmann 2008). Published clinical outcomes have reported variable results, but most studies have examined patient cohorts who had differing indications for Dynesys insertion, and often the Dynesys system was combined with another procedure (Grob 2005, Cakir 2003, Stoll 2002).

Spondylolisthesis and Stenosis
Most of the published studies looked at patients with degenerative stenosis and listhesis.

Welch et al. (2007) presented 101 such patients, who underwent decompression and Dynesys stabilisation. They reported promising outcomes at 1 year, with significant improvements in leg pain, back pain and Oswestry disability scores. Similar findings were reported by Lee et al. (20 patients with an average follow-up of over 2 years; 2009), Schnake (26 patients with a minimum follow-up of 2 years, 2006), Stoll et al. (83 patients with an average follow up of 38 months, 2002) and Schaeren (19 patients with a mean follow-up of 52 months, 2008). All 5 studies conclude that Dynesys with adequate decompression is a safe and effective alternative to conventional fusion. Fayyazi et al. (2010) looked at in vivo segmental motion by inserting tantalum beads into the vertebral bodies of 6 patients who underwent decompression and Dynesys stabilisation. Their conclusion, that Dynesys seems to stabilise degenerative spondylolisthesis, supports the findings of previous studies.

Degenerative disc disease
The mainstay of conservative treatment for degenerate lumbar discs includes physiotherapy and non-steroidal anti-inflammatory drugs. Traction and intradiscal electrothermal treatments have been shown to be ineffective. The effectiveness of other minimally invasive treatments such as intradiscal steroid injections, intradiscal radiofrequency thermocoagulation, and epidural steroid injections are also unproven (Madigan 2009, Muzin 2008). The benefits of surgical, against conservative, management have been the subject of several studies (Lee 2003, Anderson 2006), and the conclusions remain mixed.

The use of Dynesys in treating degenerate disc disease is supported by in vivo findings that glycosaminoglycan content increases at instrumented levels, whilst they decrease in non-instrumented levels. The study performed by Vaga et al. (2009) used delayed Gadolinium-Enhanced Magnetic Resonance Imaging of Cartilage 6 months after implantation of Dynesys. The authors conclude that Dynesys is able to stop and partially reverse the disc degeneration, especially in seriously degenerated discs, however, there is also incrementation of the stress on adjacent levels, where it induces a matrix suffering and an early degeneration (Vaga 2009). Putzier et al. (2005) showed, via an MRI study, that Dynesys prevents progression of initial disc degeneration after nucleotomy, however, an MRI study by Kumar et al. (2008) suggested that disc degeneration continues at both Dynesys-bridged and adjacent segments. A cadaveric study demonstrated that Dynesys reduces intradiscal pressure from neutral to extension but not to flexion compared to an uninstrumented spine. Lateral flexion showed wide variations and no significant result could be found. There was no difference in disc pressures with axial rotation (Schmoelz 2006)

The only study looking at Dynesys as the sole treatment of discogenic back pain showed significant improvements in mean back pain, SF-36 and Oswestry disability index scores, but noted wide variations in outcomes between patients in addition to significant complication rates (Lau 2011).

Degenerative Scoliosis
Only one study looked specifically at Dynesys in degenerative scoliosis. Di Silvestre et al. (2010) retrospectively reported on 29 patients with degenerative scoliosis who underwent Dynesys stabilisation combined with decompressive laminectomy. The authors reported significant improvements in clinical outcomes and concluded that Dynesys augmentation of decompressive surgery in elderly patients with degenerative scoliosis was safe, less aggressive than instrumented fusion and maintained stability.

Poor Outcomes
Not all studies have shown favourable results. Grob et al. (2005) retrospectively reviewed 31 Dynesys patients via a postal questionnaire. Within the two year follow up period, 19% of patients had required further surgery, back and leg pain remained moderately high and the patient satisfaction rate was only 50%. The authors conclude that such results were no better than lumbar fusion with respect to patient-orientated outcomes. Bothman et al. (2008) reported improved pain scores in 29 of 40 cases o patients who had Dynesys stabilisation, and noted that those who had concomitant nerve root decompression fared better. Würgler-Hauri et al. (2008) assessed 37 patients with stenosis, instability and degenerate discs. Although radicular leg pain scores improved, 30%of patients described their outcomes as fair or poor, and over 50% of patients had a Prolo Economic Status and Prolo Functional of 4 or 5. The authors conclude that the reported biomechanical principles of Dynesys do not reflect advantages in outcome compared with no stabilisation or other stabilisation systems after microsurgical radicular decompression reported in the literature.

Complications
There is a growing body of evidence of significant complication rates with Dynesys. This is especially apparent in longer-term studies, with screw loosening, breakage and adjacent segment wear being the main concerns. Loosening rates have varied from none, in a study of 29 patients with 54 month follow up (Di Silvestre 2010), to as high as 25% in a recent animal study (Cunningham 2010). Most cohorts report loosening rates of less than 2%, but the definition of “loosening” has varied between studies. Other studies have questioned the clinical importance of radiological loosening, as many patients do not note any adverse effect on clinical improvement or they remain symptom-free (Ko 2010, Kocak 2010). The issue of screw loosening has been addressed to some extent with the introduction of hydroxyapatite (HA) coated screws. A recent study compared 312 HA Dynesys screws in 58 patients to 366 non-HA Dynesys screws in 71 patients and showed an almost 10-fold decrease in loosening rates in the HA group (1 in 312 compared to 11 in 366). However, the follow-up periods between the two groups were different (Mueller 2011).

Screw breakage has also been observed and a biomechanical study comparing various dynamic stabilisation devices suggest that the cause may be related to implant design (Meyers 2008).

No studies have assessed the clinical significance of accelerated adjacent segment wear in Dynesys patients. Although biomechanical studies suggest that Dynesys significantly restricts movement at the bridged level, Cakir et al. (2009) showed that when compared to fusion, there is less restriction of both global lumbar and segmental motion. However, any efforts to look specifically at adjacent segment wear would require large numbers of patients in order to separate natural disease progression from implant-related disease.

Many studies report a significant revision rate. Though not directly related to the implant, revision surgery rates vary in the literature from 0% to 27.5%. The reasons for revision surgery mostly consisted of loose/broken/misplaced screws, adjacent segment degeneration and poor clinical outcomes. Stoll et al. documented a revision rate of 7 out of 83 (9%) at 3 years, Grob et al. had 6 out of 31 (19%) at over 2 years and Bothmann et a.l reported a 27.5% revision rate in 54 cases with a mean follow-up of 16 months. Although not being technically demanding to insert, spacer length and construct tension has been shown to be important with respect to clinical outcomes. A cadaveric study by Noisi et al. (2008)showed that spacer length affected the compression of the posterior elements, with a shorter spacer typically producing greater facets loads than a longer one. This may be an important causative factor of persisting post-operative pain, whilst the trend of deteriorating outcomes over time as described in some studies may be explained by a retrieval study which demonstrated permanent time-related spacer deformation (Ianuzzi 2010).

Conclusion
Improved fusion techniques have led to radiographically confirmed fusion rates of up to 95%, but this has failed to translate into an increase in successful clinical outcomes (approximately 70%) (Agazzi 1999, Fritzell 2001, 2002, Resnick 2005, Turner 1992). Efficacy aside, adjacent segment wear, autologous bone graft donor site morbidity and implant fatigue failure are ongoing concerns of lumbar spine fusion (Banwart 1995, Katz 1997, Sawin 1998, Kumar 2001, Markwalder 2002). Additionally, surgical alteration of the normal spine kinematics has an impact on adjacent segments, which may be a factor causing a percentage of patients to be worse after surgery. In this unhappy situation, reversal of arthrodesis is not possible, and there are limited further options. For these reasons, surgeons are rightly circumspect about fusion as a surgical strategy, and alternatives have been sought.

Pain relief is the goal of any treatment, and it is clearly preferable to achieve this outcome without abolishing movement. Flexible operations have thus been gaining favour during the past two decades. Intervertebral disc prosthesis has acquired a following, and some favourable outcomes are reported (Bertagnoli 2002, Delamater 2006) but it does require an anterior approach, with potentially hazardous surgery if there is ever a need to revisit the operative field.

Dynesys was one of the earlier posterior stabilisation systems and although there is a fair amount of literature documenting its use, the ideal indications still remain unclear. Schwarzenbach et al. (2005) predicted increased failure rates in elderly patients with osteoporotic bone or in patients with a severe segmental macro-instability combined with degenerative olisthesis and advanced disc degeneration. Lau et al. (2011) attempted to identify prognostic indicators in patients with degenerate disc disease and suggested that patient age and the number of levels requiring stabilisation are significant factors.

Various studies have cast doubt upon the ability of Dynesys to fulfil its role as a soft stabilising device, and taking into account the high reoperation rate, it is not surprising that it is losing favour to other systems.

However, there is a cohort of patients in which Dynesys produces good results. Further studies are required to clarify the exact indications and identify prognostic indicators.

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