CHAPTER 294 Lumbar Arthroplasty

Total Disk Replacement and Nucleus Replacement Technologies

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

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

The initial lumbar TDR designs generally used cobalt chrome end plates with either spike or keel fixation, as well as a polyethylene core or cobalt chrome core. These initial designs were limited to placement via the anterior retroperitoneal approach. Second- and third-generation devices have incorporated novel materials with the potential for an elastomeric component to the artificial disk function. Newer designs also allow for more flexibility in surgical approach, incorporating the standard posterior and lateral transpsoas approaches to the lumbar spine.

Charite Artificial Disc (Depuy Spine)

Cobalt chrome endplates with an unconstrained, polyethylene floating core and spike fixation. They have a three-piece design with a two-stage application and endplates followed by core placement (Fig. 294-2).

FIGURE 294-2 Charite Artificial Disc.

(Courtesy Depuy Spine.)

InMotion Artificial Disc (Depuy Spine)

The next-generation Charite device has redesigned end plates and a no-impaction application technique. The three-piece design has a single-stage application. The reconfigured cobalt chrome end plates and spike fixation have the same unconstrained, polyethylene floating core (Fig. 294-3).

FIGURE 294-3 InMotion Artificial Disc.

(Courtesy Depuy Spine.)

Prodisc-L Artificial Disk (Synthes Spine)

The Prodisc-L has cobalt chrome end plates with a semiconstrained, polyethylene core and keel fixation and a three-piece design with a two-stage application (Fig. 294-4).

FIGURE 294-4 Prodisc-L Artificial Disc.

(Courtesy Synthes Spine.)

Maverick Artificial Disk (Medtronic)

The Maverick has a cobalt chrome, metal-on-metal design with semiconstrained ball and socket design with keel fixation (Fig. 294-5).

FIGURE 294-5 Maverick Artificial Disc.

(Courtesy Medtronic.)

Kineflex Artificial Disk (Spinal Motion)

The Kineflex features cobalt chrome, metal-on-metal design with a semiconstrained core, a floating core with retention ring and keel fixation, and a three-piece design with a single-stage application (Fig. 294-6).

FIGURE 294-6 Kineflex Artificial Disc.

(Courtesy Spinal Motion.)

FlexiCore Artificial Disk (Stryker Spine)

The FlexiCore has a cobalt chrome, metal-on-metal design with a constrained domed core and spike fixation (Fig. 294-7).

FIGURE 294-7 Flexicore Artificial Disc.

(Courtesy Stryker Spine.)

Activ-L Artificial Disk (Aesculap)

The Activ-L features a ball-and-socket design, cobalt chrome end plates with a semiconstrained polyethylene core, and modular end plates available in spike or keel fixation. It has a three-piece design with a single-stage application (Fig. 294-8).

FIGURE 294-8 Activ-L Artificial Disc.

(Courtesy Aesculap.)

Nucleus Replacement

Operative Technique

The surgical approach for nucleus replacement is variable depending on the specific device. Generally, devices can be placed less invasively with multiple approach options. For example, the NUBAC mechanical nucleus and DASCOR elastomeric nucleus can each be placed via an anterolateral retroperitoneal or posterior approach at L5-S1 and a lateral transpsoas approach or posterior approach at L4-5 and above.^19–21

Nucleus Replacement Devices

Clinically, indications for nucleus replacement may be separated into two general categories. First, nucleus replacement can be used in select postdiscectomy patients as a largely prophylactic procedure to prevent recurrent disk herniations or progressive DDD.²¹ It is well documented that a small but significant proportion of patients (approximately 3% to 15%) undergoing standard microdiscectomy will develop recurrent herniations or progressive degenerative changes with symptomatic low back pain.^38–40 Iatrogenic changes seen postdiscectomy include loss of disk space height and dessication.^41,42 Partial disk replacement in the postdiscectomy setting offers the theoretical benefit of slowing future degenerative changes by maintaining disk space height and normal motion. Similar to TDR, nucleus replacement can also be used in patients with mechanical low back pain caused by mild to moderate DDD.^19,21

Functionally, nucleus replacement devices can be divided into two broad classifications: elastomeric and mechanical.^19,21 Elastomeric devices can be further subdivided into hydrogel and nonhydrogel replacements. These devices are either preformed or injectable. Injectable nucleus replacements are delivered in liquid form into the nucleotomy void and cure in situ. Preformed nucleus replacements are precured polymers (Table 294-3).^19,21,24

TABLE 294-3 Nucleus Replacement Devices

I Elastomeric nucleus 1 Hydrogel a Preformed i Prosthetic Disc Nucleus (PDN) (Raymedica) ii NeuDisc (Replication Medical) iii SaluDisc (SpineMedica) iv Geliflex (Synthes) v Aquarelle (Stryker) b Injectable i NuCore Injectable Disc Nucleus (IDN) (Spine Wave) ii Biodisc (Cryolife) iii Hydrafil (Synthes) 2 Nonhydrogel a Preformed i Newcleus (Zimmer) b Injectable i DASCOR (Disc Dynamics) ii PNR (Trans1) II Mechanical nucleus 1 One-piece a Regain (Biomet) b Satellite (Medtronic) 2 Two-piece a NUBAC (Pioneer Surgical)

Mechanical devices can be subdivided into one- and two-piece designs (Table 294-3).^19,21

Elastomeric Nucleus Replacement

The advantages of elastomeric nucleus replacement include the ability to re-create the natural function of the normal nucleus pulposus with uniform stress distribution and shock absorption capability. A central challenge involved in the use of preformed devices is implant extrusion because of their inherently deformable nature. For injectable devices, biocompatibility, long-term durability, and avoidance of leakage are of crucial importance.^16,21,22,43

Prosthetic Disk Nucleus (Raymedica)

The Prosthetic Disk Nucleus (PDN) consists of a preformed hydrogel core consisting of polyacrylonitrile and polyacrylamide with a polyethylene woven jacket. PDN was first implanted in humans in 1996. PDN remains the most widely studied nucleoplasty device worldwide.^16,22,26 PDN is implanted dehydrated and subsequently hydrates and expands. It is designed to absorb up to 80% of its own weight in water. Extrusion risk with the original design led to design and approach modification. Subsequent designs include the PDN Solo and Hydraflex (Fig. 294-9).¹⁶

FIGURE 294-9 PDN Hydraflex.

(Courtesy Raymedica.)

NeuDisc (Replication Medical)

NeuDisc is a compressible preformed hydrogel consisting of an aquacryl polymer reinforced with Dacron mesh. It is implanted dehydrated and expands anisotropically to conform to the nucleotomy defect.²¹

NuCore Injectable Disc Nucleus (Spine Wave)

The NuCore Injectable Disc Nucleus (IDN) is an injectable nonhydrogel nucleus. It is an rDNA-based synthetic protein copolymer composed of silk and elastin. NuCore has been used in both postdiscectomy and early DDD indications.²¹

Dascor Prosthetic Intervertebral Nucleus (Disc Dynamics)

Dascor is a constrained, injectable, nonhydrogel nucleus. It is applied as a two-part in situ curable polyurethane and an expandable polyurethane balloon, which is inserted into the disk space after the nucleus has been removed. The balloon is then injected under pressure with the flowable polymer that conforms to the shape and size of the disk space. The flowable polymer cures, creating a firm but pliable implant with shock absorption capability.^16,19

Mechanical Nucleus Replacement

The advantages of mechanical nucleus replacement include strength and durability. Challenges include maintaining an even stress distribution and lack of shock absorption. The major weakness revolves around a lack of anchor to the end plates, which predisposes to subsidence and expulsion.^20,21,43

NUBAC Disk Arthroplasty Device (Pioneer Surgical)

Nubac is a two-piece, mechanical nucleus. It is composed of polyetheretherketone (PEEK) and is the first PEEK-on-PEEK articulated intradiscal arthroplasty device (Fig. 294-10). The Nubac has a ball-and-socket design with a large surface contact area to distribute stress and theoretically lower the risk of subsidence. Nubac has been implanted through a lateral transpsoas approach at L3-4 and L4-5, as well as anterior retroperitoneal and posterior approaches at L5-S1.^19–21

FIGURE 294-10 Nubac.

(Courtesy Pioneer Surgical.)