Lumbar Spine Disc Replacement
Derrick G. Sueki, Erin Carr and Babak Barcohana
Etiology
Low back pain is a potentially disabling condition with a lifetime prevalence of 60% to 80% in the U.S. adult population. Pain may be due to a simple muscle sprain, sciatica caused by a disc herniation, vertebral fractures, or a number of other conditions, but degeneration of the spine is often the number one cause.
Disc degeneration in the lumbar spine has a significant effect on the functional behavior of the lumbar discs. As the proteoglycan content is lost and the osmotic pressure decreases within the nucleus, there is a diminished ability to retain water within the disc. The loss of volume leads to a reduced disc height, which commonly results in intervertebral foraminal stenosis, inappropriate stress concentrations causing osteophyte formation, and central stenosis. This causes lumbar instability, axial back pain, and radicular leg pain, which can become disabling and chronic, resulting in depression, loss of work, and the inability to enjoy simple activities of daily living.
The goals of lumbar spinal surgery are to alleviate pain, restore stability, and improve neurologic injury. Lumbar disc degeneration is a continuum with a spectrum of etiologic conditions resulting in pain. Based on where in the spectrum the patient falls, various surgical techniques and approaches will be offered. This chapter focuses on artificial lumbar disc arthroplasty.
Surgical Indications and Considerations
Patients with pain related to the lumbar spine have various complaints ranging from axial low back pain, sensation of instability, difficulty bending forward, pain with prolonged sitting or standing, inability to lift heavy items, and/or radicular leg complaints. The goal of the clinician is to identify the cause of the pain.
Various diagnostic tests are employed in the detection of the pain generator. Initially, a thorough history and physical examination are of utmost importance. The goals are to rule out other causes of pain such as infections, tumors, and visceral conditions, which may result in back pain. A careful history is taken regarding trauma, fevers, chills, weight loss, cancer history, and other associated symptoms. Questions regarding the back are also obtained, including timing of injury, aggravating factors, description of pain (sharp, dull, numbness), radicular complaints, and weakness.
Subsequently, imaging studies are obtained, including plain radiographs (Fig. 17-1), MRI studies, and CT scans. Alignment, disc heights, signs of degenerative changes, and neural compression is noted. Electrodiagnostic studies, bone densitometry, and discograms are occasionally ordered.
Once the diagnosis is established, treatment is initiated. Absent neurologic deficits and unbearable pain, nonoperative treatment is recommended. This may include physical therapy for stretching, strengthening, and modalities. Back braces may be used for short periods of time to allow the muscles to relax. Medications are prescribed, including antiinflammatory medications, pain medications, and muscle relaxants. Acupuncture, chiropractic care, physical therapy, heat, ice, traction, epidural injections, and facet injections may be ordered as well.
When the diagnosis is certain and nonoperative measures have failed, surgery is recommended. Based on the diagnosis, various surgical procedures may be recommended. For example, for an isolated disc herniation, a lumbar microdiscectomy may be considered whereas decompression and instrumented fusion may be considered for an unstable spondylolisthesis.
There are a select group of patients for which lumbar disc arthroplasty is an option. These are patients for whom an isolated decompression or discectomy is insufficient. Additionally, these patients have symptoms arising from a single lumbar disc level with complaints of axial low back pain with disc dysfunction. Based on U.S. Food and Drug Administration (FDA) guidelines, lumbar disc replacement is approved only for one level procedures. These patients would otherwise be candidates for a lumbar fusion procedure.
Criteria for disc replacement includes degenerative disc disease with discogenic back pain isolated to one level from L3-S1 in skeletally mature patients who have failed 6 months of nonoperative treatment. Contraindications include spondylolisthesis greater than grade I, significant instability or facet arthrosis, infection, bony spinal stenosis, allergy or sensitivity to implant, compromised or small vertebral bodies, isolated radicular compression syndromes, or pars defects.
As of the writing of this chapter, two lumbar artificial disc replacement devices have been granted FDA approval in the United States, including Synthes ProDisc-L (Fig. 17-2) and Charité artificial discs. These implants consist of cobalt chromium, titanium, and ultra-high molecular weight polyethylene.
The goal and potential advantages of artificial disc replacement are to maintain motion at the operated level, thereby replicating the biomechanics of the normal disc. This serves to reduce the mechanical forces that would be transmitted to adjacent segments, which are seen with rigid fusion procedures and which may lead to early adjacent segment disease and degeneration. The device would serve to anatomic disc height while maintaining structural integrity. It would need to withstand lumbar forces with long-term stability and endurance. Given that it is not a fusion device but a motion sparing device, pseudarthrosis is not a concern. However, the implant requires integration of bone into its surfaces. There is a potential concern that fusion may still occur at the operated level.
Surgical Procedure
The surgical approach for artificial disc procedures is similar to that performed for an anterior lumbar interbody fusion. Currently available lumbar disc replacement devices are placed from an anterior lumbar approach.
The patient is positioned on the operating room table in a supine position with all bony prominences well padded after induction of general endotracheal tube anesthesia (Fig. 17-3). Once the surgical level is identified with intraoperative C-arm fluoroscopic images, the skin is prepped and draped in the usual sterile fashion. A retroperitoneal approach to the spine is performed. A paramedian transverse or vertical skin incision is made. The rectus sheath is incised and the rectus muscle is retracted laterally. The posterior rectus sheath is encountered and incised to reach the preperitoneal space. The abdominal muscles, including the external oblique, internal oblique, and transverses abdominis, are divided. The transversalis fascia is then divided to allow exposure of the extra retroperitoneal space. The peritoneum and its contents are carefully retracted to allow access to the retroperitoneal space. Here, various neurovascular and visceral structures are encountered, including the ureter, genitofemoral nerve and branches, psoas musculature, aorta, vena cava, sympathetic chain, and iliac vessels (Fig. 17-4).
Injury to the sympathetic nerves in males can result in retrograde ejaculation. Deep venous thrombosis may occur in addition to injury to any of the neurovascular and visceral structures in this approach. Although many spine surgeons perform the anterior approach themselves, often general or vascular surgeons are employed to access the spine.
Once the spine is exposed and the adjacent structures are protected, a radical discectomy is performed. If necessary, neural decompression may also be performed. Next, various trials are placed to measure the size of the implant. Great care is taken to position the implant properly to maintain the appropriate center or rotation in the spine (Fig. 17-5). Multiple intraoperative images are obtained in addition to direct visualization to achieve this. The final implant is placed and is evaluated, ensuring that it is rigid with good contact (Fig. 17-6). The instruments and retractors are removed and a meticulous layered wound closure is performed.
Complications from this procedure include vascular injury, ureteral injury, wound infection, postoperative ileus, neurologic injury, dural tear, deep venous thrombosis, retrograde ejaculation, vertebral fracture, hardware failure or migration, subsidence, malpositioned implant, or fusion. The complication rate is reported to be less than 10%.
The outcomes of lumbar artificial disc replacements have been quite favorable. The results have been similar to lumbar fusion results with respect to functional outcomes and pain relief. Further research is necessary to determine whether disc replacement surgery reduces the rate of adjacent segment disease as compared with fusion procedures, but the early data are promising.
Physical therapy is key after all lumbar spinal procedures to strengthen and increase the flexibility of the spine with decreasing postoperative scar formation. Signs of infection should be watched for in the immediate postoperative period. If the patient exhibits increased pain, loss of pulses, leg pain, lower extremity swelling, or changes in neurologic examination, the physician should be contacted.
Various disc replacement products are being developed, not only for anterior approaches but also for placement through lateral or posterior approaches, which would eliminate the risks associated with the anterior approach. Research is being performed to evaluate various nucleus replacement devices to either replace or rejuvenate the nucleus of the disc. This will significantly alter the approach to and treatment of spinal related conditions.
Lumbar Disc Replacement Surgery
Therapy Guidelines for Rehabilitation
The lumbar spine can be one of the most challenging regions of the body to treat. There are many factors associated with the lumbar spine that contribute to the challenge of this region. Anatomically, the lumbar spine consists of 5 moving spinal segments and 10 articulating joints. Multiple ligaments give the region its passive stability while multiple muscles provide the active stability of the region. The nerve roots of the cauda equina run through the spinal canal in the lumbar region and exit through the intervertebral canal. These are just a few of the structural components of the lumbar spine that must work in concert to provide for pain-free and seamless movement within the region.1–3 Biomechanically, the lumbar spine is designed to provide motion as well as stability. It is a transitional zone that allows upper body motion on a relatively fixed sacrum. The sacrum in turn will transition the weight of the central axis outward into the hips and lower extremities.
The concept of replacing a lumbar disc is not new. Attempts were made in the 1950s and 1960s, but both attempts failed to produce successful results.4 In East Germany during the early 1980s, Shellnac and Buttner-Jans designed the first successful artificial disc, the SB Charité disc. Since developed, the artificial disc and surgical technique have been used in Europe, yet it wasn’t until 2004 that the SB Charité disc was even approved by the FDA in the United States. By comparison, in 1911, lumbar fusion, or arthrodesis, was first employed in the United States and is still considered the gold standard for lumbar surgery. Rehabilitation following lumbar fusion/arthrodesis has been well established. Clinical guidelines and empirical data validating rehabilitation have also been generated for the surgery. In comparison, very few clinical guidelines have been established for lumbar disc replacement surgery and no research currently exists validating any of the suggested protocols. The guidelines that follow will be a synthesis of established tissue healing guidelines, protocols for similar spinal surgeries, and treatment geared specifically for the attributes unique to lumbar disc replacement. They are not meant to replace or supplant clinical reasoning processes. Instead they are meant as a supplement to clinical reasoning and decision-making. Each patient who has undergone total lumbar disc replacement surgery is unique. The guidelines presented should be used as a point of departure from which the clinician can customize the program to the individual’s needs.
Principles of Tissue Healing5,6
A clinician must have a firm grasp of the tissue healing process if they are to effectively rehabilitate any patient. Variation exists in the categorization of healing; some clinicians prefer to use a system based upon symptom acuity. Acute symptoms are present for the first 3 weeks immediately following injury. The subacute phase begins at 3 weeks and continues until 2 to 3 months after injury. Symptoms lasting longer then 2 to 3 months are considered chronic. Conversely, other systems of classification are based upon the physiologic goal of the phase. This type of physiologic based system of classification will provide the framework of this chapter.
Phase 1 is considered the inflammation phase and is so named because of the phase’s physiologic goal of producing inflammation within the injured area. Inflammation is the body’s initial response to any injury or surgery. Immediately after surgery, the body begins the process of repair. Inflammation occurs and intensifies in the surgical region over the course of the next several days and reaches its peak production within the first 72 hours after injury. The generation of acute inflammation is generally completed within 14 days and during these first 14 days, several events occur.6,7 Clinically, rehabilitation during the inflammation phase of tissue healing should focus upon the prevention of blood loss, reduction of inflammation, and managing the pain that accompanies tissue damage.
The second phase of tissue healing is the reparative phase. The chief physiologic goal of this phase is to repair the injured tissue. Chronologically, this phase begins immediately after injury and concludes around 21 days after injury, running concurrently with the inflammation phase of healing. It is valuable for the clinician to know the exact surgical technique used by the surgeon. With the disc replacement surgery, the injured tissue is actually removed and replaced with an artificial disc. Healing of the disc is not an issue in this case. Instead reparation focuses on providing an environment of healing for the tissue that was incised in the process of replacing the disc. The surgical technique will influence the rehabilitation. The primary function of this phase is the formation of the dense connective scar tissue needed to repair the wound and reestablish structural continuity of the affected region. Most of the actual dense connective tissue development is completed by day 21. Clinically, the goal of rehabilitation in this phase should be to promote the development of the new dense connective reparative tissue.
The final phase of the healing process is the remodeling phase. The main purpose of this phase of healing is to strengthen the newly formed dense connective scar tissue. Classically, this phase is divided into two subphases, the consolidation subphase and the maturation subphase. While the purpose of the two subphases is essentially the same, they are characterized by several key factors. During the consolidation subphase, tissue is being formed and converted. Therefore, there are large quantities of fibroblast and angioblast cells present within the tissue. This subphase lasts from 22 to 60 days. Strengthening of the newly formed connective tissue should be the goal during this subphase. Care must be taken during this phase so as not to exceed the mechanical limits of the newly formed tissue, as overstress to the tissue will result in tissue injury and delayed healing. The second subphase, the maturation subphase, occurs from day 60 to 360 and is hallmarked by dense connective scar tissues that are fully fibrous in nature. For this reason, a progression in the strengthening of the affected tissues may begin more aggressively. As in the consolidation subphase, a rehabilitation programs must provide appropriate levels of stress to encourage dense connective scar tissue formation without creating or exacerbating tissue injury.
Summary Statement
Although guidelines can provide generalized timeframes for healing and recovery, it is important to realize that a firm grasp of the factors listed above will enable the clinician to individualize the rehabilitation program for each patient (also consideration is always given to the patient’s signs and symptoms). No two patients are identical. Therefore, no two rehabilitation programs should be identical. Solid clinical reasoning regarding the patient and the nature of his or her injury and surgery will ultimately drive the rehabilitation process. Table 17-1 summarizes the soft tissue healing timeframe for all three phases of healing. Adequate muscle activity and protection must accompany the healing process to progress activity levels. Healing tissues may be compromised because of increased levels of strain without adequate muscle support and protection.
TABLE 17-1
Soft Tissue Healing Timeframes
| Phase | Events | Timeframe |
| Phase I: Inflammation | Vasoconstriction in immediate area Vasodilation in surrounding areas Wound closure Removal of foreign and necrotic tissue |
0-14 days |
| Phase II: Reparative | Fibroblasts enter region to create dense connective tissue scars Angioblasts enter the region for revascularization |
0-21 days |
| Phase IIIa: Remodeling | Dense connective tissue is converted from cellular to fibrous | 22-60 days |
| Phase IIIb: Remodeling | Dense connective tissue is strengthened | 61-84 days |
| Phase IIIc: Remodeling | Dense connective tissue is strengthened | 85-360 days |
Data from Nitz A: Soft tissue injury and repair. In Placzek J, Boyce D, editors: Orthopaedic physical therapy secrets, Philadelphia, 2001, Hanley and Belfus; Frenkel S, Grew J: Soft tissue repair. In Spivak J, et al, editors: Orthopaedics: A study guide, New York, 1999, McGraw-Hill.
Attributes Unique to Lumbar Disc Replacement Surgery
The gold standard for surgical treatment of chronic low back pain is the lumbar fusion surgery. But like all surgeries, no surgical technique has 100% success rate and in the case of lumbar fusion surgery, 20% of patients will require additional surgery within 5 years after the initial surgical technique.8–12 See Box 17-1 for indicators that lumbar disc replacement surgery may be required. The most common reasons for failure of the surgery are bone graft donor morbidity, the formation of pseudoarthrosis, and adjacent spinal segment degeneration. One of the major factors believed to be associated with these failure factors is the loss of normal lumbar biomechanics following spinal fusion. The lumbar disc replacement surgery has been designed to eliminate these factors. The disc replacement is designed to maintain normal spinal biomechanics at the surgical site, decompression of the lumbar facets and neural structures, and restore the normal disc height between spinal segments.13–16 The disc replacement surgery is performed anteriorly and requires incisions through the rectus abdominis and the anterior aspect of the disc space. Following surgery, these two structures are weak and vulnerable to injury. Rehabilitation programs should address the unique aspects of this surgery and interventions designed accordingly.
Description of Rehabilitation and Rationale for Using Instrumentation
Phase I: Inflammatory Phase
TIME: Weeks 1 to 2 (Days 0 to 14)
GOALS: Protection of the surgical site, decrease pain and inflammation, initiate patient education regarding neutral lumbar spine mechanics, begin walking program (Table 17-2)
TABLE 17-2
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| Rehabilitation Phase | Criteria to Progress to This Phase | Anticipated Impairments and Functional Limitations | Intervention | Goal | Rationale |
|
Phase I |
Postoperative
|
Patient education:
Exercise:
|
|
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Hospital Rehabilitation.
Immediately following surgery, the goals while in the hospital should focus on patient education, protection of the surgical site, reduction in pain and inflammation, and restoration of independent activities of daily living. The normal hospital stay is between 5 to 7 days, with discharge either to a home environment or a skilled nursing facility.14,15
While in the hospital setting, the patient will be instructed in how to protect the surgical site. This is accomplished by instructing the patient on maintaining proper neutral spine during motion. Additionally, a lumbar stabilization brace is issued to the patient for additional support and protection. Instructions regarding the duration of its use are determined by the physician and may vary on a case-by-case basis. Log rolling and abdominal bracing techniques are used to get into and out of bed, and transitioning from supine to a sitting position. Care should be taken to avoid overstressing the abdominal muscles since the rectus was surgically incised and is subsequently weak and subject to tearing or injury. Initial examination and evaluation should include assessment of the wound, hip passive range of motion (ROM) testing, bed mobility, and gait assessment.
Hospital rehabilitation should also include gentle abdominal activation/core strengthening. The goal is not strength, but muscle recruitment. Circulation exercises are also incorporated early in the rehabilitation process. Ankle pumping exercises and thromboembolic hose hose stockings are used to prevent pooling of blood in the lower extremities. Diaphragmatic breathing exercises can be used to mobilize the abdominal muscles and abdominal contents to stimulate the lymphatic system and encourage circulation. Since the abdominal region is the site for most of the surgery, it is not uncommon for inflammation and edema to accumulate in the abdomen.
Weight-bearing activities should also begin early in the rehabilitation process. Sit to stand and gait activities should be initiated. Initially, standing and gait training will be accomplished with the aid of a front wheel walker. By the end of the hospital stay, the patient should be ambulating with the aid of a single point cane. Ambulation to and from the restroom should begin immediately with assistance as needed. These activities should progress until the patient is independent. Gentle lumbar spinal ROM can also be initiated in the hospital. Lumbar flexion exercise is the only direction of motion allowable initially. Because the disc space and abdominal cavity was incised anteriorly, it is the weakest portion of the body. Overstressing these tissues should be avoided. 
The clinician should avoid excessive and repetitive extension exercises, as well as lateral flexion and rotation. These precautions are generally in place for 6 to 8 weeks. Prone lying should also be avoided during this time period because of weakness and sensitivity of the anterior tissues.
Before discharge from the hospital, it is important that the clinician educates the patient on proper lumbar spine mechanics during activity and the need to avoid excessive trunk extension, side bend, or rotation. Refer to Box 17-2 for specific patient guidelines to follow after discharge. The patient should be advised to refrain from heavy lifting, bearing down during defecation, and abdominal splinting during coughing or sneezing. Before discharge, the need for a continued home exercise program should also be addressed. Patients can be discharged once they are able to walk unassisted or with minimal assistance depending on their postsurgical care and rehabilitation plans. They also must be free of complications, have normalized their bowel and bladder function, and show a good understanding of their surgical precautions and activity limitations.
Initial Posthospital Rehabilitation.
The second postoperative week will occur at home or a skilled nursing facility. Activities during this later stage of the inflammation phase are a continuation of the care received while in the hospital. During this time, activities should center on resuming protected normal daily activities. The patient should be encouraged to increase their daily sitting, standing, and walking tolerances. Pain and fatigue should guide the progression. The lumbar stabilization belt should be worn 24 hours a day unless otherwise ordered by the physician. Patient exercises may progress. The patient can begin gentle neutral spine lumbar stabilization exercises. Once again care must be taken to avoid overstressing the abdominal muscles. The goal is muscle recruitment, not strengthening. The patient may begin gentle lower extremity strengthening exercises, but care must be taken to stabilize the lumbar region. The clinician should keep in mind throughout this phase that the primary goal of this phase of rehabilitation is protection of the surgery, pain abatement, and restoration of protected daily activities.
Phase II: Reparative Phase
TIME: Week 3 (Days 0 to 21)
GOALS: Understand neutral spine concepts, increase lower extremity mobility, improve upright tolerance, improve protected activities of daily living, increase cardiovascular function (Table 17-3)
TABLE 17-3
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| Rehabilitation Phase | Criteria to Progress to This Phase | Anticipated Impairments and Functional Limitations | Intervention | Goal | Rationale |
|
Phase II |
As in phase I
|
Continue interventions in phase I
Exercise: |
|
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In many instances, phase II of the rehabilitation process will take place independently in the patient’s home. Home therapy is rarely indicated. Therefore, education regarding patient progression through the first month following surgery is an important aspect of hospital care. The clinician’s advice and instructions will be followed for the next 3 to 4 weeks. During the reparative phase of tissue healing, the body begins to form and lay down scar tissue at the surgical site, thus enhancing the integrity of the musculatures, ligaments, and capsule to withstand gradual increases in loads to the tissues. Therefore, as time progresses, increasing load can be placed upon the surgically repaired tissue. Rehabilitation should be a continuation of phase I and progress restoring lower extremity ranges of motion and independence with self-care skills. Movement improves circulation and prevents the formation of scar tissue adhesions between the nerve and the healing tissue surrounding the surgery. Following lumbar disc replacement surgery, scar tissue formation is inevitable in and around the surgical site. In certain instances, scar tissue can adhere to surrounding tissues, impacting mobility of any structure to which it attaches. Therefore, movement of the lower extremity and lumbar region should be encouraged to promote circulation and prevent adhesion formation. Throughout all activities and exercises, the patient should be encouraged to maintain a neutral lumbar spine. Activities should not increase symptoms. Protection of the surgical site and proper immobilization should continue until the physician has seen evidence that the prosthetic is well situated. At this time, the physician will approve additional lumbar motion and activities.
Phase IIIa: Remodeling Phase
TIME: Weeks 4 to 8 (Days 22 to 60)
GOALS: Enhance nerve healing and mobility, prevent scar tissue formation, increase lower extremity strength and endurance, improve thoracic spine and sacral mobility, begin normalization of functional daily activities, restoration of lumbar ROM (Table 17-4)
TABLE 17-4
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| Rehabilitation Phase | Criteria to Progress to This Phase | Anticipated Impairments and Functional Limitations | Intervention | Goal | Rationale |
|
Phase IIIa |
Continue with phase II interventions as needed with the following:
Exercise: |
|
|
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AROM, Active range of motion; LE, lower extremity; PROM, passive range of motion.
Between the end of the fourth week and up to the sixth postoperative week, the patient’s physician will reassess the patient. Generally, this reassessment will include a new radiographic study. Most physicians will release the patient to begin outpatient rehabilitation following this reassessment. This decision will be dependent upon several factors, including patient symptoms and function. At 4 to 6 weeks following surgery, it is anticipated that the patient will continue to have mild (possibly moderate) low back pain and achiness. This will most likely be present in the morning and at the end of the day. Functionally, the patient should be walking limited community distances with a single point cane for balance. Neurologic symptoms that are the result of spinal compression or inflammation should be improving and stabilizing. Objectively, the physician will order a spinal radiograph to assess the position of the prosthetic. If all of these factors are acceptable, the physician will allow the patient to begin outpatient rehabilitation.
The first postoperative outpatient examination should include evaluation of the patient’s scar, assessment of posture and gait, balance testing, and active range of motion assessment. ROM can be tested in all directions of motion, but end range extension, rotation, and side bending must be avoided for 6 weeks. Additionally, the clinician should conduct a neurologic examination if nerve involvement is suspected. The clinician should also assess the patient’s lumbar soft tissue, looking for muscle guarding and atrophy. Quick screening of the patient’s lower extremity should also be completed. Once the patient has been screened and deemed appropriate for phase III rehabilitation, the clinician can begin to design his or her treatment plan.
