Is Degenerative Spondylolisthesis a Contraindication for Total Disc Replacement? Kineflex Lumbar Disc Replacement in 7 Patients With 24-Month Follow-up

DISCUSSION

The etiology of degenerative spondylolisthesis (DSPL) is poorly understood. Sagittal orientation of the facet joint has been implicated as a predisposing factor²⁰ but might be instead a consequence of the remodelling taking place during development of facet arthritis.²¹ Controversy exists as to whether the pathology starts primarily in the facet joints²² or within the intervertebral disc.¹ Whatever the etiology, the result is segmental spondylolisthesis, segmental kyphosis, spinal and recess stenosis, and facet joint arthritis.

Our cohort of patients consisted of 3 different entities: degenerative spondylolisthesis proper, adjacent segment disease (ASD) with resulting (degenerative) spondylolisthesis at the adjacent segment, and 1 iatrogenic spondylolisthesis caused by an over-distracted posterior device. The resulting clinical and radiological picture is very similar. Clinical patients complained about mechanical LBP and leg pain, inability to stand or walk longer distances. Radiologically, the deformity resulted in low lumbar flat back deformities with compensatory retrolisthesis of the higher lumbar motion segments and flexion of the pelvis on lateral standing radiographs. In each of the 3 entities the resulting surgical treatment aim is similar: decompression of neural structures, stabilization of the motion segment, and restoration of the spinal balance. As we are primarily describing a surgical technique aimed to treat these different components, we considered it justifiable to sum these patients together.

Most surgical outcome studies on DSPL were retrospective and not controlled. Randomised studies have only compared surgical treatment consisting of posterolateral fusion with or without instrumentation and with posterior decompression alone.² Long-term outcome of surgical treatment seems more favorable with fusion than without fusion.^{23, 24}

Anterior lumbar interbody fusion (ALIF) surgery reliably corrects sagittal imbalance and listhetic slip in significant segmental kyphosis associated with DSPL.^{1, 6, 7} Anterior column support was recommended by Sengupta and Herkowitz for patients with Grade 2 slips or higher or when frank kyphosis is present.⁸

More recently Choi and Sung reported on a large patient group, 14 patients with DSPL, using single-level standalone rectangular cages. At 27 months follow-up there was subsidence of over 2 mm in 77% of patients and a 13% nonunion rate, but neither correlated with recurrence of symptoms.²⁵ In ALIF there is a nonunion rate which might be asymptomatic,²⁵ but in case of a later established symptomatic anterior nonunion, supplementation with an instrumented posterior fusion does not always relieve the clinical symptoms. ALIF, as a standalone procedure, has a risk of anterior dislodgement in spondylolisthesis. Furthermore, the primary contact area of fusion cages is far less than the endplate surface area of the disc prosthesis.

Brantigan et al.²⁶ reported 10-year results on a subgroup of an original US Food and Drug Administration trial, using carbon fiber posterior lumbar interbody fusion (PLIF) cages in conjunction with a posterior screw and plating system (360° fusion) with excellent clinical and radiological outcomes. McAfee et al.²⁷ reported on 120 patients with spondylolisthesis, half of them with DSPL, who were treated with a 360-degree fusion through a single posterior approach with a 98% fusion rate, good radiological outcome, and few complications.

In a prospective study, Konno and Kikuchi⁹ compared surgical decompression, together with posterior dynamic Graf ligament stabilization, to decompression surgery alone. They found better clinical outcome in the Graf ligament group. Significant clinical improvement was confirmed in other studies using dynamic stabilization,^{2, 10–12} despite an increase in facet arthrosis.¹¹ Posterior tension-band-like devices, in the absence of anterior support, should increase the load on the facet joints, and they are unable to significantly improve sagittal alignment. Taking this into consideration, the improvement in clinical outcome in these patients is remarkable. Recently McAfee et al. reported clinical improvement with a motionpreserving facet replacement system (TOPS).²⁸

Despite the success with posterior motion-preserving techniques in DSPL, even in the absence of spondylolisthesis and kyphosis reduction,^{2, 10–12} DSPL over 3 mm is considered a contraindication for TDR.²⁷ Nevertheless, objective confirmation of the outcome of TDR in DSPL is missing. Considerable reduction of the spondylolisthesis should be desirable in order to restore the normal 3-joint anatomy (disc and 2 facet joints) of the motion segment. This could not be achieved with posterior motion-preserving procedures.^{2, 10–12}

The potential advantages of disc replacement with an unconstrained disc and non-rigidly held prosthetic endplates during insertion are, at least in theory, the following (see also Figure 1 and Hähnle et al.¹³:)

1. During the insertion process the prosthetic endplates can follow the shape of the bony endplates as they are able to pivot around the mechanism. The leading edge of a rectangular cage, or rigidly held disc prosthesis, is more likely to be impacted into the bony endplate, possibly resulting in endplate fracture and subsidence.

2. Once the Kineflex disc prosthesis is almost lying flush with the posterior vertebral body wall of the cephalad vertebra, the inferior endplate can be further advanced to facilitate reduction of the spondylolisthesis. As during this process part of the impact of the further advancement is absorbed in the prosthetic mechanism, the cephalad prosthetic endplate stays behind, avoiding protrusion of the superior prosthetic endplate into the spinal canal. With a cage or an unconstrained disc prosthesis, this manuever would easily result in impaction of the implant into the posterior part of the vertebral endplate of the cephalad vertebra (endplate fracture) or in protrusion of the implant into the spinal canal.

3. During mobilization of the patient the forces that could lead to expulsion of the inferior prosthetic endplate are at least partially dampened within the prosthetic mechanism, which allows translation but at the same time is recentering. Furthermore, flexion/extension motion through the operated segment does not lead to cyclical loss of contact (rocking) of the implant/bone surfaces and possible nonunion or nonintegration, as the contact is maintained by the adaptive movement of the prosthetic endplate.

With the Kineflex insertion technique used, we achieved excellent sagittal balance and slip correction in all patients. The continuous advancement of the inferior prosthetic endplate, after seating the prosthesis close to the posterior edge of the apical vertebral endplate, allowed additional reduction of the spondylolisthesis. The postoperatively high translational forces on the inferior prosthetic endplate could be counteracted with additional screw fixation used as a buttress. No postoperative hypermobility developed at the replaced levels. One reoperation was performed early after the index procedure with excellent clinical and radiological final outcome (Figure 4).

The most common level in DSPL is the L4-5 motion segment. At this level dislodgement of the implant would be of particular concern because of the proximity of important vascular structures. We would generally try to avoid anterior revision surgery at this level after 17–18 days and would rather wait past 3 months when the postoperative inflammation has completely settled. We therefore performed our anterior revision surgery for partial endplate extrusion early, after the first follow-up at 8 days postoperatively, despite good clinical relief of symptoms at that early stage (Figure 4).

The indications for TDR have hardly been modified since the onset of modern type TDR over 20 years ago.²⁹ Only recently Siepe et al.³⁰ published clinical outcome of TDR dependent on indications. There is a need for arthroplasty surgeons to publish results of off-label TDR procedures, in order to establish their validity for different indications and in order to learn from the experience of other surgeons.