Lumbar Arthroplasty

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CHAPTER 294 Lumbar Arthroplasty

Total Disk Replacement and Nucleus Replacement Technologies

Domagoj Coric, Paul Kim

Mechanical low back pain (LBP) because of lumbar degenerative disk disease (DDD) has been problematic to both diagnose and treat. Nonsurgical treatment is successful in the majority of patients with LBP.¹^–³ Unfortunately, a significant minority of patients at various stages of the degenerative cascade remain debilitated with diskogenic LBP. Surgical treatment for diskogenic LBP has traditionally focused on fusion with concomitant loss of function of the motion segment.^4,⁵ Traditional lumbar fusion techniques have evolved over the years. The advent of increasingly sophisticated spinal instrumentation and implants have consistently increased fusion rates.^6,⁷ Unfortunately, although fusion rates have approached 100%, clinical success rates have lagged significantly behind.^4,⁸ Arthrodesis of the lumbar three-joint functional spinal unit (anterior disk and two posterior facet joints) results in loss of motion with a known incidence of adjacent level symptomatic degeneration ranging from 10% to 30%.⁹^–¹⁵ Spine arthroplasty techniques allow for pain relief by removing presumed pain generators, the annulus and/or nucleus, while maintaining function (i.e. motion with uniform stress distribution) and, thereby, theoretically, protecting adjacent levels.

Partial disk replacement dates back to the 1960s, when Fernstrom implanted stainless steel balls into the cervical and lumbar spine.¹⁶ The modern era of lumbar arthroplasty was ushered in when Butner-Janz and Schellnack developed the original Charite artificial disk in the early 1980s.¹⁷ In the late 1980s, Marnay developed the Prodisc-L.¹⁸ Subsequently, total disk replacement has seen a steady evolution with a proliferation of devices and concomitant refinement in implant design, surgical technique, and instrumentation.

Lumbar arthroplasty can be divided into total disk replacement (TDR) and subtotal/partial disk replacement (PDR) or nucleus replacement. TDR devices can be categorized according to their composite biomaterials (metal-on-metal or metal-on-polymer), biomechanics (unconstrained, semiconstrained, constrained), components (one-, two-, or three-piece designs) or fixation (spike or keel). Constrained devices have a mechanical stop within the range of physiologic motion, while semiconstrained devices have a mechanical stop outside the range of physiologic motion. Unconstrained devices lack a mechanical stop.

Nucleus replacement devices represent an even more heterogeneous group of devices and are earlier in the developmental process. Functionally, nucleus replacement devices can be divided into two broad classifications: elastomeric and mechanical (Fig. 294-1).^16,¹⁹^–²¹

FIGURE 294-1 Classification of lumbar disk arthroplasty devices.

The primary indication for both mechanical nucleus replacements and total disk replacement (TDR) is mechanical low back pain because of DDD.⁵ Elastomeric nucleus replacement can also be used in the post-discectomy setting.²¹

Generally, both types of nucleus replacement devices represent therapeutic interventions aimed at an early stage in the degenerative cascade (mild to moderate DDD). Advantages of nucleus replacement include minimally invasive and multiple approach options, including anterior retroperitoneal, lateral, and posterior approaches. Limited exposure and annulotomy allow for multiple revision options including TDR and fusion.^19,^20,²²^–²⁶ Challenges of nucleus replacement include migration or expulsion risk (devices generally are not fixed to end plates) and subsidence.^21,²⁷^–²⁹

TDR is generally applicable for more advanced DDD (moderate to severe). Advantages of TDR include more definitive near-total disk removal that addresses pathology involving both the annulus and the nucleus as well as reliable fixation to bony end plates.³⁰^–³³ Challenges of TDR include implant longevity, failure, and wear debris, as well as more challenging revision strategies (Table 294-1).³⁴^–³⁷

TABLE 294-1 Comparison of Nucleus Replacement and Total Disk Replacement

NUCLEUS REPLACEMENT	TOTAL DISK REPLACEMENT
Limited annulotomy; nuclectomy only	Wide annulotomy; near-total discectomy
Mild to moderate DDD	Moderate-to-severe DDD
Anterior, lateral, posterior approach	Primarily anterior retroperitoneal approach
No end plate fixation	End plate fixation
Limited device surface area	Large device surface area

Total Disk Replacement

Operative Technique

The general operative technique for various TDR devices is similar. A standard anterior retroperitoneal approach is followed by exposure of the anterior disk. In the United States, exposure is generally accomplished with an access surgeon, either general or vascular. The midline should be preliminarily identified and marked using anatomic landmarks before extensive dissection. Proper identification and verification of the midline is crucial to proper artificial disk placement. It is imperative to mobilize the iliac vessels to visualize the lateral margins of the anterior disk. A wide annulotomy is followed by total Discectomy. Discectomy is facilitated by mobilizing the disk with interbody distractors and releasing or resecting the posterior annulus and the posterior longitudinal ligament (PLL). Disk removal is accomplished with standard technique using variously sized and angled curets and pituitary and Kerrison rongeurs. Care is taken to remove the cartilaginous end plates while preserving the bony end plates to minimize the risk of implant subsidence. Attention is paid to complete disk removal, retaining only the lateral annulus bilaterally. Special care must be taken with lateral disk resection where any retained disk material can be pushed into the foramen during artificial disk placement. The artificial disk is then placed under anteroposterior (AP) and lateral fluoroscopic guidance using an individualized combination of sizing, trialing, midline verification, and disk placement.

Total Disk Replacement Devices

The Food and Drug Administration’s (FDA) approval of the Charite Artificial Disc in October 2004 ushered in the era of spinal arthroplasty in the United States.¹⁷ Subsequently, Prodisc-L (2006) and the next-generation Charite device, InMotion (2007), received FDA approval.³⁵ Several more devices have followed (Maverick, Kineflex, FlexiCore, and Activ-L) with a number of others in various stages of development (Table 294-2). Cumulatively, these studies have produced a substantial body of level 1 data from prospective randomized studies confirming TDR’s efficacy in the treatment of symptomatic lumbar DDD.^17,^18,^36,³⁷

TABLE 294-2 Total Disk Replacement Devices

DEVICE MANUFACTURER TYPE
METAL-ON-POLYMER
Charite	Depuy Spine	Unconstrained
InMotion	Depuy Spine	Unconstrained
Prodisc-L	Synthes Spine	Semiconstrained
Activ-L	Aesculap	Semiconstrained
Mobi-L	LDR Spine	Unconstrained
Triumph	Globus	Posterior
Physio-L	Nexgen	Elastomeric
CAdisc	Ranier Tech	Elastomeric
eDisc	Theken	Elastomeric
Dynardi	Zimmer	Semiconstrained
Freedom	AxioMed	Elastomeric
METAL-ON-METAL
Maverick	Medtronic	Semiconstrained
Kineflex	Spinal Motion	Semiconstrained
FlexiCore	Stryker Spine	Fully constrained
Lateral Disc	NuVasive	Lateral approach
TrueDisc PL	Disc Motion Tech	Antenia posterior