Continuing Education Activity

Diskectomy is a surgical procedure that is performed to remove the herniated or damaged portion of the intervertebral disk to alleviate pressure and relieve symptoms on the spinal nerve roots associated with lumbar disk herniation. This surgical intervention is commonly performed to alleviate pain, weakness, numbness, and other neurological symptoms caused by compression of spinal nerve roots. Diskectomy involves accessing the affected disk through a small incision in the back and carefully removing the extruded disk material, thereby relieving pressure on the adjacent nerve structures. This surgical procedure can significantly enhance a patient's quality of life and functional outcomes, boasting high success rates in appropriately selected cases. Initial management usually involves conservative treatments, with surgery reserved for cases unresponsive to these measures.

This activity encompasses essential topics such as patient selection criteria, preoperative evaluation, surgical approaches, intraoperative considerations, postoperative care protocols, and potential complications associated with diskectomy. This activity allows clinicians to gain comprehensive knowledge and skills related to the diagnosis, management, and surgical techniques involved in diskectomy procedures. In addition, this activity facilitates clinicians' understanding of the anatomical, pathophysiological, and biomechanical principles underlying lumbar disk herniation and diskectomy procedures. This understanding empowers clinicians to improve patient care and outcomes and satisfaction while expanding their expertise in spine surgery.

Objectives:

• Identify appropriate patients for diskectomy based on clinical presentation, medical history, diagnostic imaging findings, and response to conservative management.

• Assess the effectiveness of conservative treatments prior to considering diskectomy, including medication management, physical therapy, and spinal injections.

• Apply evidence-based guidelines and best practices in the perioperative management of patients undergoing diskectomy to minimize complications and optimize surgical outcomes and patient safety.

• Collaborate with interdisciplinary healthcare teams to optimize patient care throughout the surgical process and until their functional recovery.

Introduction

Diskectomy is a surgical procedure performed to remove the herniated or damaged portion of the intervertebral disk to alleviate pressure and symptoms on the spinal nerve roots associated with lumbar disk herniation. This surgical intervention, commonly used in cases of spinal disk herniation, addresses the protrusion of the disk's inner core (nucleus pulposus) through a tear or weakness in its outer layer (annulus fibrosus), which can cause symptoms such as pain, numbness, weakness, or other neurological symptoms resulting from compression of spinal nerve roots.

Lumbar disk herniation represents a prevalent and early manifestation of lumbar spine degeneration, with reported incidences ranging from 2% to 3% and a prevalence of approximately 12%.[1][2] Among individuals aged 35 or older, the prevalence is 4.8% in men and 2.5% in women.[2] Lumbar disk herniation typically occurs at the L4-L5 and L5-S1 levels, often requiring surgical intervention and accounting for a significant portion of spinal surgeries.[3][4][5] Initially, management approaches typically involve conservative treatments, including oral medications, rest, and physical therapy, with surgical interventions reserved for cases unresponsive to conservative measures. Physical therapists are critical in administering the gold-standard first-line treatment. Their profound understanding of human mechanics and therapeutic modalities enables the successful treatment of many disk herniations without surgery.[6]

The historical evolution of diskectomy techniques highlights significant milestones in spinal surgery. Diskectomy involves accessing the affected disk through a small incision in the back and carefully removing the extruded disk material, thereby relieving pressure on the adjacent nerve structures. While the core principle of diskectomy surgery, aimed at alleviating nerve impingement, remains constant, newer surgical strategies prioritize minimizing trauma to the multifidus muscle and enhancing surgical visualization. This emphasis on reduced tissue disruption and improved visualization underscores advancements aimed at optimizing patient outcomes and minimizing postoperative complications.[4]

Beginning with Mixter and Barr's description of laminectomy via the L3 to sacrum approach for lumbar disk herniation in 1934, subsequent innovations have revolutionized surgical approaches. In the 1970s, hemi-laminectomy emerged, followed by Caspar and Williams' introduction of microdiskectomy using a 3-cm incision in 1977.[7] Wiltse and Spencer delineated the paraspinal approach for managing extraforaminal disks in 1988, coinciding with Kambin and Sampson's pioneering fully endoscopic approach.[7] In 1993, Mayer and Brock introduced tubular retractors. Foley and Smith revolutionized the field with the microendoscopic diskectomy technique in 1997, using a video-assisted approach through a 2-cm incision, significantly minimizing tissue disruption.[7][8] 

Various surgical strategies are currently used for diskectomy, including open, minimally invasive open lumbar diskectomy, microlumbar diskectomy, microendoscopic diskectomy, and fully endoscopic diskectomy.[5][9] These surgical procedures can significantly enhance a patient's quality of life and functional outcomes, with high success rates observed in appropriately selected cases. Later in this activity, these methods will be further elaborated upon, highlighting their respective approaches and benefits in treating disk herniation. 

Anatomy and Physiology

A diskectomy is a surgical procedure that addresses a damaged or herniated intervertebral disk within the spine. The spinal column, or the vertebral column or backbone, comprises individual vertebrae stacked atop each other, offering structural support to the body and safeguarding the spinal cord. Intervertebral disks, positioned between adjacent vertebrae, act as shock absorbers, reducing impact during movement. They consist of a tough outer layer called the annulus fibrosus and a gel-like inner core known as the nucleus pulposus. Nerve roots extend from the spinal cord, exiting through intervertebral foramina, facilitating the transmission of sensory and motor signals.

A herniated disk develops when the nucleus pulposus protrudes through a tear or weakness in the annulus fibrosus, potentially irritating nearby nerve roots and resulting in symptoms such as pain, numbness, tingling, or weakness. Experts believe these symptoms arise due to a multifactorial mechanism, including noxious stimuli to the disk material, an inflammatory cascade affecting local nerve roots, and direct compression of nerve roots by extruded disk material.

Indications

Indications for diskectomy include:

• Cauda equina syndrome
• Refractory and disabling pain despite 6 to 8 weeks of conservative management
• Progressive or new onset neurological deficits [4] 

Contraindications

The following conditions are contraindications to diskectomy:

• Bony spinal canal stenosis
• Concurrent segmental instability
• Malignant tumors with dural involvement
• Neurological or vascular pathologies mimicking disk herniations
• Local or systemic infection
• A vertebral fracture that requires fusion or instrumentation 

These are not absolute contraindications and may vary based on individual patient factors and surgeon preference. In some cases, despite contraindications, a surgeon may determine that the potential benefits of a diskectomy outweigh the risks, especially if conservative treatments have proven ineffective or if the patient's symptoms significantly impact their quality of life. Thus, the decision to undergo surgery should be thoroughly discussed between the patient and their healthcare provider to ensure alignment with the patient's goals and preferences while minimizing risks.

Preparation

Several prerequisites and preparations are essential to ensure optimal outcomes before undergoing a diskectomy. Firstly, a detailed history is taken, and a thorough neurological examination is conducted to identify specific symptoms and signs indicative of disk herniation, such as a step page with pelvic glissading for L5 disk involvement and foot-dragging for S1 disk involvement. In addition, a detailed dermatomal map must be obtained to locate the affected nerve roots accurately.

Additionally, correlation with pertinent imaging studies, including x-ray, computed tomography (CT) scans, magnetic resonance imaging (MRI), and diskography, helps confirm the presence, level, and pattern of the herniated disk (posterolateral, foraminal, extraforaminal, or extruded versus sequestrated).[4] A protrusion occurs when the height of the herniation is less than the length of the base. Extrusion refers to when the size of the base is less than the height of the herniation. Sequestration describes a scenario without continuity between the herniated material and the intervertebral disk.[2] 

Patient counseling is another crucial aspect, involving discussions on the natural history of the disease. Typically, the natural history of lumbar disk herniation entails rapid symptom relief over a period of 4 to 6 weeks, with recurrence rates ranging from 5% to 10%, regardless of the management strategy used.[2] Notably, spontaneous resorption of the herniated disk material occurs in up to 66.66% of cases, contributing to symptom improvement and resolution over time.[10] Various management and surgical strategies and the risks and benefits of surgical intervention should also be conferred.[4]

Proper anesthesia selection, whether spinal or general anesthesia, is determined based on individual patient factors and preferences, with both options demonstrating comparable outcomes.[11] However, in a study, patients undergoing general anesthesia showed significantly improved visual acuity scale scores regarding postoperative leg and back pain compared to those who received local and epidural anesthesia during microendoscopic diskectomy. The same study revealed that local and epidural anesthesia minimized costs and risks.[12]

Surgical aspects include adherence to safety protocols, maintaining a sterile environment in the operating theater, administering antibiotic prophylaxis, and ensuring proper positioning. This involves placing the patient in the genupectoral position to free the abdomen and provide chest support for stability. Eye protection should also be implemented. The shoulders should be abducted to 90°, while the elbows should be flexed to 90°, creating a kyphotic curve in the spine. This facilitates the opening of the inter-laminar space and provides optimal access to the surgical site. Lastly, localization and exposure techniques, guided by surface landmarks and fluoroscopy, ensure accurate targeting and access to the affected disk space, facilitating surgical intervention.

Technique or Treatment

The gold standard of surgical intervention for lumbar disk herniation is diskectomy or removal of disk material contributing to symptomatology. Lumbar diskectomy has a success rate of 60% to 90%.[5] Although historically performed through an open approach, several minimally invasive surgical options have become available, providing surgeons with multiple surgical options.  

Open Diskectomy 

After administering general anesthesia, the patient is positioned prone on a spine frame, such as the Wilson or Allen Bow, or a dedicated table. Transverse pads are strategically placed at the iliac crest and chest to allow for hip flexion, increasing the interlaminar distance while minimizing pressure on the abdomen to reduce central venous pressure. Palpation of bony landmarks, including the sacrum and iliac crests corresponding to the L4-L5 disk level, assists in determining the starting point and trajectory of the surgical approach. Following appropriate sterile skin preparation, localization using a spinal needle and fluoroscopic control confirms the target level, and a 3- to 4-cm longitudinal incision is marked at the midline, centered around the radiographic marker.

The skin incision is initiated with a sharp scalpel, and subcutaneous dissection using electrocautery exposes the lumbar fascia. This fascia is then incised off the midline, guided by palpation of the spinous processes on the side corresponding to the affected disk pathology. Ensuring that the fascia spans the interspinous distance at the target level is essential. A radiographic marker may confirm the spinal level and the cranially directed trajectory in line with the interspinous space, as observed on a lateral fluoroscopic image. Subsequent dissection involves a subperiosteal elevation of paraspinal musculature from the superior and inferior spinous processes down to the laminar junction, performed with electrocautery. Lateral dissection continues bluntly with a Cobb elevator until reaching the facet joint, taking care not to disrupt its capsule. Notably, the crucial aspects of the procedure include maintaining clear visualization of the interlaminar space, removing dissected muscle tissue from the surgical field, and ensuring proper retraction and meticulous hemostasis using electrocautery.

At this stage, the surgeon releases the ligamentum flavum from its attachment on the anterior aspect of the lamina of the superior vertebra using a curette. An angled Woodson elevator may be cautiously inserted anterior to the ligamentum, directed caudally, to protect the dura mater underneath. Subsequently, the ligamentum flavum is sharply incised to facilitate its retraction with a Penfield elevator, thereby enabling visualization of the exiting nerve root and associated epidural fat. Resection of the medial aspect of the inferior facet of the superior vertebra may be necessary to achieve adequate exposure. A Penfield or blunt probe is inserted into the neuroforamen to mobilize the root, facilitating its medial retraction. With clear visualization of the intervertebral disk space, fragmented or herniated tissue may be meticulously removed using pituitary rongeurs.

In cases where a portion of the herniated disk remains beneath the posterior longitudinal ligament, the surgeon may need to utilize a scalpel to incise the annulus for access. The epidural space must be meticulously probed in all directions using a Woodson elevator to thoroughly remove any additional disk or ligamentous tissue. Furthermore, irrigating the disk space with saline via a bulb syringe is recommended to expel any loose disk fragments that may have escaped visualization. Achieving meticulous hemostasis via bipolar electrocautery is paramount, followed by liberal irrigation of the wound with saline. Closure of the fascial and subcutaneous layers is performed using absorbable sutures, with the skin closure method determined based on the surgeon's preference.

Microlumbar Diskectomy 

Using intraoperative radiography to verify the target level, a 2- to 3-cm longitudinal midline incision mark is made over the interspace. The incision is then carefully created with a sharp scalpel, and subcutaneous dissection with electrocautery exposes the lumbar fascia along the midline. The muscular aponeurosis is incised just off the midline on the approach side, and subperiosteal release of the multifidus muscle from the spinous process is performed using a Cobb elevator, extending out to the facet joints. This dissection should encompass half of the lamina above and below the interspace while avoiding violating the facet capsule. Repeat imaging is conducted to confirm the correct level, after which retractors are inserted to establish the working window, and the microscope is positioned accordingly over the incision. Alternatively, the surgeon can use magnifying loupes.[13]

The next step involves exposing and releasing the ligamentum flavum from its attachment on the anterior aspect of the superior lamina of the inferior vertebra using a curette. This ligamentum is then sharply incised to allow for retraction, with an angled Woodson elevator used beneath it to protect the dura during the process. Retraction or removal of the ligamentum, often done with a Kerrison rongeur, facilitates visualization of the exiting nerve root alongside its associated epidural fat. Identification of the nerve root is crucial before proceeding with disk resection.

In cases of inadequate visualization, removal of the medial aspect of the inferior facet of the superior vertebra may be required. Preserving at least half of the facet joint and 8 to 12 mm of bone from the lateral edge of decompression to the edge of the pars interarticularis is essential to minimize the risk of iatrogenic instability. A laminotomy of the inferior portion of the upper lamina may also be necessary for off-centered disk locations. Following nerve root mobilization with a blunt ball-tipped probe and retraction with a nerve root retractor, disk excision is performed by removing the fragmented or herniated tissue.[13]

When a portion of the disk remains behind the posterior longitudinal ligament, incising the annulus may be necessary for its removal. Up- and down-facing curettes and pituitary rongeurs are commonly used for this diskectomy procedure. Care must be taken to avoid violating anteriorly beyond the disk space, mainly when microscopic assistance is used, to prevent injury to major vessels. Following disk removal, a blunt instrument such as a Penfield dissector or Woodson elevator ensures nerve and dural sac freedom. This involves probing in all directions to confirm adequate decompression and check for any remaining disk or ligamentous tissue.[13]

Following disk removal, the disk space is irrigated with saline using a hollow flexible tube or bulb syringe to ensure any unnoticed loose disk fragments are expelled. Hemostasis is meticulously achieved using bipolar cautery, and the surgical site is thoroughly irrigated with saline. Optionally, vancomycin powder can be applied to the wound before closure for antimicrobial prophylaxis. Closure begins with suturing the fascia of the lumbar musculature and subcutaneous layers using absorbable sutures, followed by skin closure according to the surgeon's preference. In cases where disk herniations extend through the foramen, a combined midline and lateral approach may be necessary for optimal access and removal.

Minimally Invasive Surgery Tubular Diskectomy

The patient is positioned and prepped as previously outlined, ensuring proper alignment and sterile conditions. A longitudinal surgical incision of approximately 1.5 to 2.0 cm is marked 1.5 cm off midline on the affected side. A #15 scalpel is used to create a stab incision, allowing for the introduction of a guide pin or K-wire, which is then advanced under lateral fluoroscopy to ensure accurate depth and docking to the lamina cranial to the affected level. Upon confirmation, the entire skin incision is made, and a fascial incision is centered over the wire. Sequential dilator retractors are then inserted to create a working channel, which may be anchored to the operating table for stability. Magnifying surgical loupes or intraoperative microscopes enhance visualization throughout the procedure. Specialized instruments designed for a tubular approach facilitate the remainder of the diskectomy, ensuring precise and efficient tissue removal. Meticulous hemostasis is achieved, followed by removing the tubular retractor system. Closure of the subcutaneous tissue and skin completes the surgical case.

Microendoscopic Diskectomy 

The patient is positioned and prepared as previously outlined, ensuring proper alignment and sterile field maintenance. Starting from a designated point 1 to 2 cm off midline ipsilateral to the pathology, the surgical team uses a spinal needle to approach the anatomical space known as the Kambin triangle at the target level. A small skin incision, typically 5 to 10 mm long, is made to access this space. Guided by fluoroscopy, successive cannulated dilators are introduced to create an 18 mm operating canal gradually.

An endoscope is inserted through this canal to visualize the disk space and the traversing and exiting nerve roots. Specialized instruments, including curettes, rongeurs, drills, and bipolar electrocautery, are passed through the canal to perform targeted decompression of nerve roots and removal of herniated disk material. A laminotomy of the cranial vertebra may be performed to enhance visualization and access. After the completion of the procedure, the single small endoscopic incision is closed with a subcuticular suture for optimal wound healing.[4]

Full Endoscopic Diskectomy 

This procedure is performed using a lateral and transforaminal approach, typically entering the spine at a point approximately 12 to 14 cm off the midline, at an angle between 20° and 30°.[4] This procedure offers a minimally invasive option for accessing and treating disk pathology. However, it is noteworthy that this approach may not be suitable for addressing sequestrated intracanal fragments and should be used judiciously based on patient presentation and imaging findings.[4]

Postoperative Care

Following a diskectomy, postoperative care primarily entails monitoring patients for any signs of complications and ensuring effective pain management. Although some patients may be discharged from the hospital as early as the first postoperative day, others may require a longer stay for physical therapy or optimized pain control. Notably, outpatient diskectomy procedures have been described and are implemented in some medical centers.[14]

External bracing is usually not necessary for spinal stability after the surgery. Patients are generally advised to gradually resume light activities within 2 weeks, routine activities within 6 weeks, and strenuous labor or contact sports after 12 weeks to minimize the risk of reherniation. However, evidence suggests that a more prompt or immediate return to unrestricted activity may yield equivalent outcomes without increased reherniation rates.[15][16] 

Rehabilitation is a crucial component of postoperative care, with activities gradually reintroduced over time.[4] In a study, preoperative rehabilitation was found to be conducted in only 35% of centers, while postoperative rehabilitation was performed as an inpatient in all centers and as an outpatient in 82% of centers.[17] Enhanced recovery after surgery (ERAS) is a surgical rehabilitation protocol gaining recognition for its potential to achieve optimal clinical outcomes.[1][18]

Complications

Possible complications of lumbar diskectomy include the conditions mentioned below.

• Dural tear: Durotomy presents a significant risk during surgery due to unintentional tearing of the dura mater. This can lead to cerebrospinal fluid leakage and subsequent complications such as meningitis. Incidental durotomy occurs in up to 9% of cases and is frequently linked to excessive nerve traction during disk exposure.[19][20][21][22] Management typically involves placing a sponge pattie and extending the laminectomy edges to expose the tear, followed by primary repair using 5-0 nonabsorbable sutures. Additionally, utilizing biological glue can assist in sealing the dural tear and reducing the risk of cerebrospinal fluid leakage.[23]

• Iatrogenic neuropraxia: Iatrogenic neuropraxia, involving direct intraoperative nerve root injury, is reported to affect 1% to 2% of cases.[24] 

• Surgical site infection: Surgical site infection occurs in 2% to 3% of cases, with wound dehiscence occurring in 1% to 2%. 

• Epidural bleeding: Epidural bleeding can be managed by compressing the venous 'lakes' with a sponge pattie. In addition, hemostatic agents are recommended.

• Vascular injury: The reported incidence is less than 1%. The aorta and inferior vena cava are particularly at risk in the L1-L4 region, while the iliac vessels are vulnerable at L4-L5 and L5-S1.[25] In cases of large vessel injuries, embolization or laparotomy may be necessary for management.

• Epidural hematoma [26]

• Disk not identified: The correct surgical level must be confirmed by reviewing the imaging studies, ensuring they are no more than 2 months old. If necessary, the surgical corridor may need to be extended to facilitate better visualization and identification of the targeted disk.

• Failed back surgery syndrome: Recurrence rates of 3% to 15% and instability rates of 20% during 10 years of follow-up have been observed.[27] Patients may have persistent pain following lumbar disk surgery, which may be a part of failed back surgery syndrome.[28]

• Postoperative diskal pseudocyst [29]

• Iliac arteriovenous fistula [30]

• Retained nonabsorbable hemostatic material [31]

• Lumbar disk herniation recurrence: The reported incidence ranges from 3% to 15%.[4] Most recurrences occur at the same level of herniation and on the same side.[32] Recurrence rates after diskectomy vary widely, ranging from 1% to 25%.[19][20][21] In a study, the recurrence rate at 5-year follow-up was 6.27%, and 63% occurred within 6 months. Modic changes, disk height index, and facet orientation show a significant correlation.[33] Modic type-II and contained disks have higher odds of recurrence.[34] Male gender, smoking status, heavy labor, obesity, and diabetes are significant predictors of recurrence.[35][36] 

• Reoperation: In 1 study comprising 1850 patients, 130 patients underwent reoperation and were successful in 62%. Herniation at different levels, recurrences at the same level, and scar formation showed excellent results in 98%, 54%, and 38% of cases.[37]

An example of such complications is demonstrated in a meta-analysis from 1997 to 2020, where various diskectomy approaches, including open, microlumbar, microendoscopic, and fully endoscopic techniques, were associated with the following:

• Recurrence rates of 4.1%, 5.1%, 3.9% and 3.5% respectively.
• Reoperation rates of 5.2%, 7.5%, 4.9%, and 4% respectively.
• Wound complication rates of 3.5%, 3.5%, 1.2%, and 2% respectively.
• Durotomy rates of 6.6%, 2.3%, 4.4%, and 1.1% respectively.
• Neurological complication rates of 1.8%, 2.8%, 4.5%, and 4.9% respectively.
• Nerve root injury rates of 0.3% of microlumbar cases, 0.8% of microendoscopic cases, and 1.2% of full endoscopic cases.[8]

Clinical Significance

The adoption of endoscopic techniques in treating lumbar disk herniation has shown promising results, generally leading to a reduction in immediate postoperative disability and hospital stay compared to minimally invasive surgery and open techniques. Although additional long-term studies are needed to determine the superiority of these methods, each approach offers unique advantages.[38] The long-term implications of the relatively minimal surgical exposure necessitated by minimally invasive surgery or endoscopic approaches have been topics of debate.[39][40][41] 

Comparative studies have demonstrated similar clinical outcomes between minimally invasive surgery and standard diskectomy procedures.[7] However, although percutaneous techniques allow for rapid recovery, they present with higher odds of recurrence and revision rates, as well as increased costs, compared to microendoscopic and standard diskectomies.[7]. Endoscopic approaches, known for reduced muscle damage, may not significantly differ in certain muscle parameters compared to conventional microdiskectomy.

Despite their benefits, minimally invasive techniques carry a higher risk of recurrence and iatrogenic complications, with increased odds of intraoperative nerve root injury.[32] While minimally invasive surgery requires a significant learning curve, it has shown advantages such as shorter hospital stays and lower infection rates.[4][8] A recent systematic review from the Cochrane database comparing open versus minimally invasive diskectomy provided low-quality evidence indicating lower infection rates and shorter hospital stays for minimally invasive techniques. In addition, a possibility of inferior improvement in lower back and leg pain exists compared to an open diskectomy.[42] However, the long-term implications of minimal surgical exposure remain under debate, warranting further investigation into the efficacy and safety of these approaches. Surgical preferences among spine surgeons vary, with a majority favoring microdiskectomy and early mobilization after surgery, underscoring the importance of individualized patient care in optimizing outcomes.[5] Overall, diskectomy remains a vital intervention for alleviating symptoms associated with lumbar disk herniation, with evolving techniques offering improved outcomes and patient care.

Enhancing Healthcare Team Outcomes

Effective care delivery during the preoperative, intraoperative, and postoperative phases of diskectomy relies on collaboration among various healthcare professionals. Physicians, advanced practitioners, nurses, pharmacists, and other members of the healthcare team play crucial roles in ensuring patient-centered care, optimizing outcomes, enhancing patient safety, and improving team performance. Physicians and advanced practitioners leverage their clinical expertise to assess patients, determine surgical candidacy, and develop individualized treatment plans. Nurses are pivotal in patient education, preparing patients for surgery, and providing postoperative care, including monitoring for complications and managing pain. Pharmacists contribute by ensuring appropriate medication management, including pain management and prophylactic antibiotics, while identifying potential drug interactions or contraindications.

Physical therapists and other members of the rehabilitation team play vital roles in the nonsurgical management of disk herniations, utilizing modalities such as medication, rest, and physical therapy, as well as in the comprehensive care of patients undergoing diskectomy procedures. They contribute significantly to preoperative preparation, postoperative rehabilitation, and long-term recovery, ensuring optimal patient outcomes. Effective interprofessional communication is essential throughout the care continuum, facilitating information exchange, care coordination, and shared decision-making among healthcare team members. By collaborating closely and leveraging each team member's unique skills and expertise, healthcare professionals can optimize patient outcomes, enhance patient safety, and improve team performance related to diskectomy procedures.