Metastatic spinal disease (MSD) is a rapidly growing clinical problem that adversely affects patients’ quality of life leading to significant morbidities including pain and neurological deficits. Nearly 70% of patients with systemic cancer have secondary involvement of the spinal column.
Pain arising from neural compression, skeletal instability or from cancer is usually the presenting symptom and can be severely debilitating. Apart from pain, disability can also arise from neurological deficits, occurring in 5–14% of patients. Traditionally, surgery has been indicated in conjunction with medical treatment in specific situations or when radiotherapy and/or chemotherapy have failed. It is also commonly employed in emergent situations such as rapid neurological deterioration or pathological fracture. Surgical practices have evolved over the last 100 years due to advances in surgical techniques and have been equally influenced by the advances in radiotherapy and chemotherapy. In the traditional posterior decompressive laminectomy for MSD with neurological deficits, access to the anterior tumour is limited by the inability to retract the spinal cord intraoperatively, thereby compromising the effectiveness of surgery. In the past, without instrumented stabilisation, this resulted in spinal instability, meaning that radiotherapy became the favoured treatment for MSD as laminectomy outcomes were not superior in terms of neurological function and pain outcomes.
Newer surgical approaches have evolved, using instrumentation with decompression and providing a stable spine. To make surgical management an appealing choice in MSD, surgical morbidity needs to be kept as low as possible, especially when planning oncological treatment around surgery. This is where we believe minimally invasive surgery is successful. In reducing morbidity and minimising tissue trauma, a minimally invasive approach allows the earlier introduction of postoperative radiotherapy and chemotherapy, thereby lengthening the honeymoon period—the period after surgical decompression before the residual tumour can re-expand in size to reproduce symptoms of cord compression—of surgery.
Percutaneous pedicle screw fixation (PPSF) is one of the most common modalities in minimally invasive surgery. With more and more surgeons preferring a posterior approach and the emergence of new techniques to tackle anterior column pathology with a posterior approach, the importance of PPSF is increasingly being recognised and its popularity is growing among spine surgeons. The earliest description of percutaneous fixation of lumbar spine came from Magerl who used an external fixator. Percutaneous pedicle screw fixation of the lumbar spine was first described by Mathews and Long, in which plates were used as longitudinal connectors, and Lowery and Kulkarni subsequently described the use of rods as longitudinal connectors. These earlier descriptions involved placement of the longitudinal connectors either externally or superficially, an approach that carried several potential disadvantages such as soft tissue irritation by the implant. This also resulted in a poor biomechanical advantage from long pedicle screws which led to higher incidence of implant failure and non-union.
The CD Horizon Sextant Spinal system, which is a “minimal access spinal technology”, evolved as a result of improvisation on the previous technique. Here, the instruments, especially longitudinal connectors, are placed in the anatomical position, ie. a sub-muscular position, with better biomechanical advantage and less soft tissue irritation, thus avoiding the need for routine implant removal. Technically, lateral to medial screw trajectory is better achieved with a PPSF technique than with a conventional technique due to the absence of wall of paraspinal muscles impeding enough lateral retraction. Initially introduced for degenerative spine diseases, the technique has evolved rapidly since the late 1990s.
Minimally invasive approaches have been reported to be as successful as open techniques for lumbar decompression with elimination of midline skin incision and significant paraspinal muscle dissection and retraction, resulting in reduced intraoperative blood loss, shorter operative time, reduced opioid dependence, shorter hospitalisation and earlier return to work. However, at one year postoperatively, there were no differences in clinical or radiological outcomes. Thus, minimally invasive surgery can only be shown to be advantageous in the early postoperative period, which is the period of particular interest in MSD patients with limited life expectancy. In the prospective study conducted by our group, we analysed 27 MSD patients who underwent PPSF. It was shown that improved functional outcomes and quality of life can be observed even with patients with predicted poor prognostic scores on survival prognostic scoring systems (Tokuhashi, Tomita).
Despite its significant benefits, surgeons’ response to minimally invasive surgery has been mixed due to the steep learning curve and some doubts as to whether effective decompression can be achieved through small incisions. In a study by Mannion et al, a single surgeon performed 50 consecutive minimally invasive lumbar spinal decompressions for spinal stenosis, showing that the operative time was initially much longer compared to an open procedure, but improved as the experience of the surgical team increased showing that the learning curve with this technique is acceptable. Spinal surgical site infection, which has been quoted up to 2% in open surgery, has been reduced by nearly 10-fold via a minimally invasive approach.
To conclude, minimally invasive surgery is beneficial in a well-selected group of MSD patients suffering from clinically significant instability with intractable back pain or motor deficits. The introduction of a minimally invasive approach can be a game-changer in the treatment of MSD because it lowers perioperative morbidity and enables earlier radiotherapy and/or chemotherapy. Having established itself in the management of degenerative spinal conditions, minimally invasive surgery is increasingly being recommended for MSD.
Naresh Kumar, Department of Orthopaedic Surgery, National University Health System, Singapore. Karthikeyan Maharajan, National University Health System, Singapore, assisted with this manuscript
1) Kumar N, Zaw AS, Reyes MR, et al. Ann Surg Oncol. 2015; 22(5): 1604–11.