Clinical Studies: Lumbosacral Fusion
Lumbosacral Fusion: Cages, Dowels, Pedicle Screws: Part 4
Many authors have reported excellent clinical results with the use of threaded cylindrical devices for ALIF. In a prospective, multicenter trial of the BAK device, Kuslich et al60 reviewed 947 patients. An anterior approach was used in 591 operations with 93% obtaining fusion at 24 months postoperatively. Pain was eliminated or reduced in 84%. Function was improved in 91%. Major complications occurred in 2%. Implant migration occurred in 1.2% with all requiring re-operation. Vessel damage or iliac vein tears (1.2%) were all repaired without apparent long-term problems. The overall rate of device related reoperation was 4.4% with most requiring additional posterior instrumentation to relieve ongoing pain. There were no instances of implant fracture or other forms of structural failure. There were no deaths, major paralyses, or deep infections. Fusion rate at 12 months after ALIF was 88.3%. At 24 months, the fusion rate increased to 93% of ALIF procedures and at 3 years after surgery (118 patients), 98.3% of patients had fused operative segments.
Blumenthal et al5 also found a low revision surgery rate (3.3%) among their series of 130 consecutive stand-alone open and laparoscopic threaded interbody cage patients. However, in a prospective non-randomized study of 51 stand-alone open and laparoscopic BAK patients, O'Dowd et al73 recently reported an overall failure rate requiring revision of 31% due to clinical failures at a mean of 15 months. Furthermore, 75% of their patients had residual symptoms at 2 years postoperatively and 47% had the same or poor self-assessment. The authors believe the unacceptable failure rate and poor clinical results were due to use of the cage as a stand alone device. Based on these findings, in order to avoid such poor outcomes, the authors recommend supplemental posterior stabilization for all threaded interbody ALIF patients. 73, 75
In the only direct comparison of threaded bone dowels and titanium cages to date, 100 anterior interbody fusion patients were randomized in a prospective study comparing titanium interbody cages (BAK) with threaded cortical bone dowels (MD II). 88 At 12 months, there were no significant differences in clinical outcome or radiographic evaluations.
Ray83 reported a large series (236) treated with his threaded cages, however, through a posterior approach. Of 208 followed for a minimum of 24 months, 203 (96%) had radiographic evidence of fusion. Clinical outcome as described by Prolo was excellent for 84(40%), good for 53(25%), fair for 44(21%), and poor for 30(14%). Unlike traditional posterolateral fusion or PLIF techniques, ALIF avoid "fusion disease": resultant postoperative muscle fibrosis, as well as muscle and facet joint denervation. This posterior fusion disease causes severe damage to the posterior spinal musculature, not only by the direct dissection but also by the denervation that must inevitably occur as the result of the destruction of its nerve supply during the exposure.
Posterior lumbar muscles are injured after posterior lumbar spine surgery, as demonstrated by findings on histology, computed tomography, and magnetic resonance imaging. These pathologic changes likely contribute to poor clinical outcome. Degeneration of the back muscle occurs just after surgery and the muscle in most reoperated patients shows severe histologic damage, including denervation, reinnervation, and early aging. External compression by a retractor increases the intramuscular pressure and decreases local muscle blood flow. The pathologic condition of the back muscle beneath the retractor blade is similar to that of skeletal muscle beneath a tourniquet.
Metabolic changes and microvascular abnormalities occur. A pathogenic mechanism for the muscle injury is based on compression and ischemia of the affected muscle. Two hours of continuous retraction caused significant histologic changes and neurogenic damage including degeneration of the neuromuscular junction and atrophy of the muscle. 52 In an animal model, muscle injury after surgery was related to the retraction time and the pressure load generated by the retractor. 53 Posterior surgical intervention to the lumbar spine always produces a risk of back muscle injury. Degeneration of the multifidus muscle was found after surgery 55 and human back muscle in patients who underwent repeat surgery showed severe neurogenic damage. 54 This muscle injury after posterior surgery might cause postoperative low back pain and compromise the functional integrity of the muscle. 80 Rantanen et al80 also found selective type 2 muscle fiber atrophy and pathologic structural changes in the back muscles of the patients who had severe handicap after posterior lumbar surgery.
Furthermore, an ALIF procedure does not significantly alter the rate of development of adjacent level degenerative changes over that of natural history. 30 In one study with 16-year follow-up after ALIF, the rate of adjacent level degenerative changes was similar to an age-matched control population. 93 Luk et al62 found no increased compensatory motion in the transition zone immediately above an ALIF. Penta et al78 concluded that the rate of degenerative changes adjacent to an ALIF at 10 years, as assessed by MRI was not significantly increased.
The advantages of anterior lumbar fusion in comparison to posterior lumbar interbody fusion are many, including ease of dissection, reduced operative time and blood loss, noninterference with the potentially painful posterior elements of the lumbar spine, and avoidance of scarring within the spinal canal. In addition, the disc can be resected in its entirety, advantageous from a structural and biochemical perspective. Pain from a degenerative disc can remain despite a solid posterolateral fusion, which is resolved with an anterior discectomy and fusion. 97
In an independent review of a prospective comparative series of anterior interbody fusions and posterolateral fusions with pedicle screw and plate fixation, 38 ALIFs did better despite a lower fusion rate. Although the fusion rate for anterior interbody fusion was less than that for posterolateral fusion with internal fixation, there was no difference in the subjective opinion of fusion between the two groups. Patients treated with ALIF were statistically significantly better in regards to functional outcome as assessed by the Low Back Outcome Score. One surgeon performed all procedures, and there was a minimum follow-up period of 2 years. Posterolateral lumbar fusion with pedicle screw instrumentation (135 patients) was compared to a group of 151 patients who underwent anterior lumbar interbody fusion. The improved outcome in the anterior fusion group, despite the higher pseudarthrosis rate, supports the concept that part of the benefit with anterior fusion is removal of the pain source itself. Another possible explanation is that some of the patients' continuing pain and disability is related to the effects of posterior surgery on the spinal musculature and the presence of a rigid pedicle screw fixation system.