a b c Case Study 1 A 3-year-old boy presented for evaluation and management of a progressive congenital scoliosis. He was diagnosed with a cardiac murmur at birth and subsequent echocardiogram revealed severe congenital cardiomyopathy and pulmonary hypertension that eventually required surgical intervention. AP and lateral radiographs ( a, b) of the spine reveal a par- tially segmented, incarcerated hemivertebra at the thoracolumbar junction. Cobb angle, measured from endplate to endplate, was 37 degrees at the time of surgery. Physical examination and MRI revealed no other neurologic findings. The patient underwent an anterior hemiepiphysiodesis and posterior hemiarthrodesis on the convex side of the curve ( c). Segmental vessels were ligated with surgical clips. The intervertebral disc, and therefore the growth potential on the concave side of the curve, were left intact. The patient tolerated the procedure well and achieved a solid arthrodesis on the convexity of the curve. The remaining growth potential produced unilateral growth and progressive correction of the curve. At latest follow-up ( d, e) the congenital curve had been reduced to 20 degrees over a 5-year period. Congenital Scoliosis Chapter 25 703 a b c d e f Case Study 2 A 14-year-old male with congenital scoliosis presented with a 55-degree upper left thoracic curve. He was otherwise neurologically intact. There were no other members in his family with scoliosis. The remaining medical work-up and MRI was negative for associated cardiac, genitourinary, or neurologic malformations. Because of the location of the congenital anomaly in the high thoracic spine, the patient developed a fairly dramatic clinical deformity with an elevated left shoulder ( a, b) and coro- nal imbalance ( c). As a result, he underwent an instrumented posterior spinal fusion. Intraope- ratively, the left convex rod was inserted first and a compression maneuver performed. The second concave rod was placed in situ with minimal distraction. A progressive loss of neuromonitoring signals prompted a Stagnara wake-up test which revealed that the patient had no voluntary motion of the lower extremities. The patient was placed back under anesthesia and both rods were loosened returning the curve to its original position. The patient was able to move all four extremities on the repeat wake-up test. The rods were locked in situ without any correction. Postoperatively, the patient was neurologi- cally intact and demonstrated a mild improvement in his clinical ( d) and radiographic appearance (e, f). This case empha- sizes the dangers associated with curve correction in the surgical treatment of the congenital curve. 704 Section Spinal Deformities and Malformations Spinal Osteotomies The selective use of asymmetric spinal osteotomies can help correct deformities in multiple planes, but must be planned carefully Most spinal osteotomies are based on a combination of two traditional osteoto- mies: the Smith-Peterson and the pedicle subtraction osteotomies. Both tech- niques were originally described for the management of flexion deformities that occurred in rheumatoid and ankylosing spondylitis patients and have since been extensively modified [35, 39, 41]. Frequently, as in patients with unsegmented bars, an asymmetric osteotomy aimed at addressing the specific vertebral anom- aly should be designed as necessary. A thin-slice or spiral CT scan is essential for preoperative surgical planning, which can be performed through either a single posterior approach or a combined approach. The inherent neurologic risks of such techniques must be well understood before undertaking such a procedure. Placement of segmental instrumentation for provisional stabilization prior to completing the osteotomy can help to reduce the risk of uncontrolled translation of the spine with corresponding neurologic injury. Hemivertebra Resection Hemivertebra at the lumbosacral junction causing an oblique take-off maybebesttreated with hemivertebra resection This procedure is done either through a posterior approach only (Fig. 4), or through a sequential or simultaneous anterior and posterior approach [7, 9, 16, 19, 20, 21, 28, 32, 33, 37]. The ultimate surgical approach selected depends on the location of the hemivertebra, its type, whether it is segmented or not, and famil- iarity of the surgeon with the technique. These procedures usually provide an average of 25°–30° of correction, with some correction of the associated kypho- sis. Perhaps the best indications are a fully segmented hemivertebra located at the lumbosacral junction associated with an oblique take-off and pelvic obliquity ( Case Introduction). Recent publications tend to show that hemivertebra resec- tion is safe even in the thoracic spine; however, they are clearly more dangerous to perform and should only be carried out by experienced spine surgeons [16]. After hemivertebra excision, the correction can be achieved and maintained by a variety of methods. Depending on the size of the patient, 4.5-mm AO screws inserted into the pedicles with a tension band system can be used, and supra- or infralaminar hooks with cast or brace treatment are also options [3]. In older patients a classic pedicle screw rod system is indicated. Depending on the size and location of the vertebra, anterior instrumentation is also an option [33]. Spinal Column Resection Spinal column resection may be the only way to rebalance the spine in patients with complex deformities In very complex spinal deformities the only way to rebalance the spine may be through a spinal column resection with shortening of the spinal column. This was described by Bradford and Tribus, and consists of an anterior approach where one or several vertebrae are removed after a decorticated osteoperiosteal flap has been elevated [6]. The involved vertebral bodies are removed down to thedura,theconvexpediclesareremoved,andasmuchaspossibleoftheconcave pedicles is removed. The posterior surgery, done in the same sitting or a few days later, consists of removing the corresponding posterior laminae and the rest of the concave pedicles. The spinal deformity is then corrected at the same time as the shortening is carried out. Careful monitoring of the neurologic function is mandatory during these exceptional procedures [6]. This procedure should be undertaken by only the most experienced spine surgeons, and only after careful preoperative planning and discussion with the patient and family. Congenital Scoliosis Chapter 25 705 ab cd Figure 4. Techniques of hemivertebra resection (posterior only) a During the posterior excision of the hemivertebra, the appropriate level is identified and pedicle screws are inserted above and below the malformation. b Next the inferior facets of the hemivertebra and the vertebra above are removed and a complete laminectomy is performed at the level of the hemivertebra exposing the neural structure. c Decancella- tion of the vertebral body of the hemivertebra is performed with a curette. The exiting nerve root is protected during this stage of the procedure by the medial pedicle wall. Discectomies above and below the hemivertebra are performed. The hemivertebral excision is completed after removal of the pedicle and the remnant of the vertebral body. This is per- formed with minimal retraction of the neural elements. d Compression with the pedicle screw rod system results in immediate correction of the deformity. Notice that after the hemivertebra is excised, two nerve roots exit through a sin- gle foramen and should be checked for possible nerve root compression. 706 Section Spinal Deformities and Malformations Miscellaneous Surgical Techniques Halo Traction The use of halo traction should be exceptional in congenital scoliosis, and it may be dangerous for neurologic function. Its use is formally contraindicated if there is a rigid acute component of kyphosis associated with the scoliosis. However, in selected cases it may be a helpful adjunct, especially in order to prepare the patient for surgery, in cases of severe respiratory compromise, or in between staged surgery [2, 38, 46]. The Rib Expander Intheappropriatepatient, the use of halo traction, the titanium rib expander, and the subcutaneous growing rod are acceptable surgical options The rib expander (Fig. 5) – the titanium rib expansion project developed in San Antonio by Campbell – will allow some spine growth as well as chest and lung expansionifcarriedoutbeforetheageof8years,torecruitmorepulmonary alveoli [10]. Its best indications are in cases of congenital scoliosis associated with fused ribs and/or patients with thoracic insufficiency syndrome and/or chest hypoplasia. Subcutaneous Rods Subcutaneous rods without fusion and subsequent lengthening may play a role in maintaining the growth of the spine in very young children, but these procedures do not address the area where the malformation of the spine is. They may be combined with convex growth arrest [12]. They expose the patient to multiple lengthening operations and carry a significant risk of complications, mostly infections or instrument complications. a Figure 5. Alternative treatment options for congenital scoliosis In carefully selected cases the use of a the titanium rib expander or Congenital Scoliosis Chapter 25 707 bc d Figure 5. (Cont.) b–d the subcutaneous growing rod is a reasonable option for the treatment of congenital anomalies of the spine. Recapitulation Epidemiology. Thetrueincidenceofcongenital scoliosis is unknown. There do not appear to be any significant ethnic or geographic differences, al- though there is a greater female to male ratio (1.4–2.5 to 1). Most cases are non-hereditary. Cases with a syndromic association can have a he- reditary component with a 10% risk to siblings and subsequent generations. Pathogenesis. In sporadic cases, the etiology is be- lieved to be an insult to the fetus during the 4th– 6th week of gestation. As a result, up to 60 % of pa- tients with congenital scoliosis may have malfor- mations in other organ systems. Classification. The congenital anomalies are classi- fied as either failure of formation, failure of seg- mentation,ormixed. Examples of failure of forma- tion are hemivertebra and wedge vertebra, while unilateral unsegmented bars and block vertebra are examples of failure of segmentation. In addi- tion, hemivertebra is further classified as fully, par- tially, or non-segmented and as incarcerated or non-incarcerated. In general, a non-incarcerated fully segmental hemivertebra has a worse progno- sis for progression compared to an incarcerated non-segmented vertebra. Clinical presentation. Often the diagnosis of the spinal deformity is made at the time of the prenatal ultrasound examination or is discovered incidental- ly. Otherwise, the child will be referred for the eval- uation of a spinal deformity. Physical findings. Examination should include the skin and spine, but one should also look for any foot or leg asymmetry, craniofacial malformations, Klip- pel-Feil web neck, and cardiac and urinary malfor- 708 Section Spinal Deformities and Malformations mations. A thorough neurologic examination is required. Diagnostic work-up. The best X-rays are usually ones taken at birth. Several Cobb angles should be calcu- lated, one within the deformity and one over the whole curve. The same landmarks should be used during subsequent measurements. A 10-degree increase in the Cobb angle is considered as progres- sion. Occasionally, although the Cobb angle does not change, the clinical deformity may worsen requiring early surgical intervention. When further detail is needed, cone down views and CT reconstructions can provide additional detail. MRI evaluation of the spinal column is mandatory. Furthermore an ultra- sound examination of the genitourinary and cardiac system should be performed as indicated. Non-operative treatment. Observation may be considered for non-progressive balanced curves. Bracing in most instances is ineffective in congeni- tal scoliosis. Natural history and progression. The rate of pro- gression in congenital scoliosis is directly related to: (1) the potential for asymmetric growth and (2) the location of the vertebral anomaly. Depending on the location, early surgical intervention may be required to address congenital curves that result in significant shoulder, pelvic, or trunk imbalance. Operative treatment. The goal is to achieve a solid fusion and prevent further progression, to achieve as straight a spine as possible at the end of growth. Prophylactic surgical procedures refer predomi- nantly to in situ fusions and hemiepiphysiodesis. The general principle is to balance the growth by slowing or stopping the convex side growth while allowing the remaining concave growth potential to catch up. Posterior spine fusion without instru- mentation and correction with a cast is an option in young children, but exposes the spine to the crankshaft phenomenon. Posterior spine fusion with instrumentation is indicated in older patients. Anterior and posterior spine fusion with instru- mentation can achieve a significant correction; however, neurologic complications are a concern. The use of spinal cord monitoring and/or a wake- up test is strongly recommended. In selected cases an osteotomy with subsequent corrective instru- mentation is an option; however, the inherent neu- rologic risks of such techniques must be well understood before undertaking such a procedure. Hemivertebra resection is done either through a posterior approach only or through a sequential or simultaneous anterior and posterior approach, and provide an average of 25°–30° of correction. Fully segmented hemivertebra at the lumbosacral junc- tion may be the best indication for resection. In very complex deformities the only way to rebal- ance the spine may be through a spinal column resection. In the appropriate patient, the use of halo traction, the titanium rib expander,andthe subcutaneous growing rod are acceptable surgi- cal options. Key Articles Wynne-Davies R (1975) Congenital vertebral anomalies: etiology and relationship to spina bifida cystica. J Med Genet 12:280 – 88 In a study of 337 patients with congenital spinal anomalies, the author found that an iso- lated hemivertebra or similar localizing defect was sporadic with no risk to subsequent siblings or offspring. Patients with multiple anomalies, however, carry a 5–10% risk to subsequent siblings. McMaster MJ, Ohtsuka K (1982) The natural history of congenital scoliosis. A study of two hundred and fifty-one patients. J Bone Joint Surg Am 64(8):1128 This paper provides a review of over 200 patients who were observed past the age of 10 without treatment. They found that final severity depended on the type of vertebral anomaly, the location of the anomaly, and the age of the patient at diagnosis. Bradford DS, Heithoff KB, Cohen M (1991) Intraspinal abnormalities and congenital spine deformities: a radiographic and MRI study. J Pediatr Orthop 11:36 –41 Forty-two patients with congenital spinal deformity were studied by MRI. Sixteen patients (38%) had an associated intraspinal abnormality. The authors recommend MRI in patients with congenital spinal deformities undergoing spinal stabilization. Congenital Scoliosis Chapter 25 709 Key Articles Roaf R (1963) The treatment of progressive scoliosis by unilateral growth arrest. J Bone Joint Surg Br 45:637 One of the earliest descriptions of the use of convex growth arrest for addressing congeni- tal scoliosis. Convex growth arrest is achieved by anterior and posterior convex fusions resultingincontinuedconcavegrowthwithpotentialcurvecorrection. Bradford DS, Tribus CB (1997) Vertebral column resection for the treatment of rig id coronal decompensation. Spine 22:1590 –9 Twenty-four patients with rigid coronal decompensation underwent anterior-posterior vertebral column resection, spinal shortening, with posterior spinal instrumentation and fusion. Average correction of coronal and sagittal plane deformity was 82 % and 87% respectively. Although the complication rate was nearly 60% (14 patients), all patients ratedtheirresultsaseithergoodorexcellent. Lazar RD, Hall JE (1999) Simultaneous anterior and posterior hemivertebra excision. Clin Orthop Rel Res 364:76 –84 Eleven patients underwent simultaneous anterior and posterior resection of a congenital hemivertebra with deformity correction using posterior instrumentation. Preoperative curves measuring an average of 47 degrees corrected to an average of 14 degrees at 28monthsfollow-up.Therewasonetransientlegweaknesswhichresolved.Nolongterm complications were noted. References 1. Akbarnia BA, Heydarian K, Ganjavian MS (1983) Concordant congenital spine deformity in monozygotic twins. J Pediatr Orthop 3:502 2. Arlet V, Papin P, Marchesi D (1999) Halo femoral traction and sliding rods in the treatment of a neurologically compromised congenital scoliosis: technique. Eur Spine J 8:329 –31 3. Arlet V, Odent T, Aebi M (2003) Congenital scoliosis. Eur Spine J 12:456–63 4. Beals RK, Robbins JR, Rolfe B (1993) Anomalies associated with vertebral malformations. Spine 18:1329–1332 5. Benacerraf BR, Greene MF, Barss VA (1986) Prenatal sonographic diagnosis of congenital hemivertebra. J Ultrasound Med 5:257–9 6. Bradford DS, Tribus CB (1997) Vertebral column resection for the treatment of rigid coronal decompensation. Spine 22:1590–9 7. Bradford DS, Boachie-Adjei O (1990) One-stage anterior and posterior hemivertebral resec- tion and arthrodesis for congenital scoliosis. J Bone Joint Surg Am 72:536–40 8. Bradford DS, Heithoff KB, Cohen M (1991) Intraspinal abnormalities and congenital spine deformities: a radiographic and MRI study. J Pediatr Orthop 11:36– 41 9. Callahan BC, Georgopoulus G, Eilert RE (1997) Hemivertebral excision for congenital scoli- osis. J Pediatr Orthop 17:96–9 10. Campbell RM, Vocke AK (2003) Growth of the thoracic spine in congenital scoliosis after expansion thoracoplasty. J Bone Joint Surg Am 85:409–20 11. Cantu JM, Urrusti J, Rosales G, et al. (1971) Evidence for autosomal recessive inheritance of costovertebral dysplasia. Clin Genet 2:149 12. Cheung KM, Zhang JG, Lu DS, et al. (2002) Ten-year follow-up study of lower thoracic hemi- vertebrae treated by convex fusion and concave distraction. Spine 27:748–53 13. Chirpaz-Cerbat JM, Michel F, Berard J, et al. (1993) Early and semi-early surgery for scolio- sis caused by hemivertebrae – indications and results. Eur J Pediatr Surg 3:144–53 14. Hall JE, Herndon WA, Levine CR (1981) Surgical treatment of congenital scoliosis with or without Harrington instrumentation. J Bone Joint Surg Am 63:608–619 15. Hattaway GL (1977) Congenital scoliosis in one of monozygotic twins: a case report. J Bone Joint Surg Am 59:837 16. Holte DC, Winter RB, Lonstein JE, et al. (1995) Excision of hemivertebrae and wedge resec- tion in the treatment of congenital scoliosis. J Bone Joint Surg Am 77:159–171 17. Keller PM, Lindseth RE, DeRosa GP (1994) Progressive congenital scoliosis treatment using a transpedicular anterior and posterior convex hemiepiphysiodesis and hemiarthrodesis. A preliminary report. Spine 19:1933–9 18. Kieffer J, Dubousset J (1994) Combined anterior and posterior convex epiphysiodesis for progressive congenital scoliosis in children aged < or =5 years. Eur Spine J 3:120–5 710 Section Spinal Deformities and Malformations 19. Klemme WR, Polly DWJ, Orchowski JR (2001) Hemivertebral excision for congenital scolio- sis in very young children. J Pediatr Orthop 21:761–4 20. Lazar RD, Hall JE (1999) Simultaneous anterior and posterior hemivertebra excision. Clin Orthop 364:76–84 21. Leatherman KD, Dickson RA (1978) Two stage corrective surgery for congenital deformity of the spine. J Bone Joint Surg Br 61:324–328 22. Loder RT (2003) Congenital scoliosis and kyphosis. In: DeWald RL, Arlet V, Carl AL, et al. (eds) Congenital scoliosis and kyphosis. New York: Thieme, pp 684–693 23. Loder RT, Urquhart A, Steen H, et al. (1995) Variability in Cobb angle measurements in chil- dren with congenital scoliosis. J Bone Joint Surg Br 77:768–70 24. Loder RT, Hernandez MJ, Lerner AL, et al. (2000) The induction of congenital spinal defor- mities in mice by maternal carbon monoxide exposure. J Pediatr Orthop 20:662–666 25. MacEwen GD, Winter RB, Hardy JH (1972) Evaluation of kidney anomalies in congenital scoliosis. J Bone Joint Surg Am 54:1451–54 26. McMaster MJ, David CJ (1986) Hemivertebra as a cause of scoliosis: a study of 104 patients. J Bone Joint Surg Br 68:588–95 27. McMaster MJ, Ohtsuka K (1982) The natural history of congenital scoliosis: a study of two hundred and fifty one patients. J Bone Joint Surg Am 64:1128–47 28. Nakamura H, Matsuda H, Konishi S, et al. (2002) Single-stage excision of hemivertebrae in the posterior approach alone for congenital spine deformity: follow-up period longer than ten years. Spine 27:110–5 29. Peterson HA, Peterson LF (1967) Hemivertebrae in identical twins with dissimilar spinal columns. J Bone Joint Surg Am 49:938 30. Rimoin DL, Fletcher BD, McKusick VA (1968) Spondylocostal dysplasia. A dominantly inherited form of short trunked dwarfism. Am J Med 45:948 31. Rothenberg S, Erickson M, Eilert R, et al. (1998) Thoracoscopic anterior spinal procedures in children. J Pediatr Orthop 33:1168–70 32. Ruf M, Harms J (2002) Hemivertebra resection by a posterior approach: innovative opera- tive technique and first results. Spine 27:1116–23 33. Shen FH, Lubicky JP (2004) Surgical excision of the hemivertebra in congenital scoliosis. J Am Coll Surg 199:652–3 34. Shen FH, Herman J, Lubicky JP (2003) A radiographic classification for identifying Klippel- Feil patients at increased risk for developing clinically significant cervical symptoms. In: 31st Annual Meeting of the Cervical Spine Research Society. Scottsdale, Arizona 35. Shen FH, Samartzis D, Jenis L, et al. (2004) Evaluation and surgical management of the rheu- matoid neck. Spine J 4:689–700 36. Shen FH, Samartzis D, Herman J, et al. (2006) Radiographic assessment of segmental motion at the atlantoaxial junction in the Klippel-Feil patient. Spine 31:171–177 37. Shono Y, Abumi K, Kaneda K (2001) One-stage posterior hemivertebra resection and cor- rection using segmental posterior instrumentation. Spine 26:752–7 38. Sink EL, Karol LA, Sanders J, et al. (2001) Efficacy of perioperative halo-gravity traction in the treatment of severe scoliosis in children. J Pediatr Orthop 21:519–24 39. Smith-Peterson MN, Larson CB, Aufranc OE (1945) Osteotomy of the spine for correction of flexion deformity in rheumatoid arthritis. J Bone Joint Surg Am 27:1–11 40. Sturm PF, Chung R, Bomze SR (2001) Hemivertebra in monozygotic twins. Spine 26:1389–91 41. Thomasen E (1985) Vertebral osteotomy for correction of kyphosis in ankylosing spondyli- tis. Clin Orthop 194:142–152 42. Thompson AG, Marks DS, Sayampanathan SR, et al. (1995) Long-term results of combined anterior and posterior convex epiphysiodesis for congenital scoliosis due to hemivertebrae. Spine 20:1380–5 43. Winter RB (1983) Congenital deformities of the spine. New York: Thieme-Stratton 44. Winter RB, Moe JH, Eilers VE (1968) Congenital scoliosis: a study of 234 patients treated and untreated. Part I: natural history. J Bone Joint Surg Am 64:1128–47 45. Winter RB, Moe JH, Lonstein JE (1983) A review of family histories in patients with congeni- tal spine deformities. Orthop Trans 7:32 46. Winter RB, Moe JH, Lonstein JE (1984) Posterior arthrodesis for congenital scoliosis. An analysis of the cases of two hundred and ninety patients five to nineteen years old. J Bone Joint Surg Am 66:1188–97 47. Wynne-Davies R (1975) Congenital vertebral anomalies: etiology and relationship to spina bifida cystica. J Med Genet 12:280–88 Congenital Scoliosis Chapter 25 711 26 Degenerative Scoliosis Max Aebi Core Messages ✔ The average age of patients with degenerative scoliosis is in the sixties ✔ Degenerative scoliosis is a form of adult scolio- sis ( =scoliosis after bony maturity) ✔ Degenerative scoliosis can be distinguished into primary (de novo) degenerative scoliosis and secondary degenerative idiopathic scolio- sis (primary curve or compensatory curves) ✔ Degenerative scoliosis can progress with time ✔ The cardinal symptoms are back pain, claudica- tion symptoms, neurological deficit and curve progression ✔ Cosmesis does not play an important role ✔ Patients with back pain in degenerative scoliosis need to be individually evaluated for surgery ✔ Clinical signs and symptoms as well as comor- bidities determine the extent of surgery ✔ The primary goal of the treatment is not curve correction but the control of back pain and claudication symptoms ✔ A decompression at the apex of the curve needs to be stabilized and fixed in order to pre- vent curve progression ✔ The loss of lordosis is often the main reason for back pain, and sagittal realignment is crucial ✔ The fixation of the lumbosacral junction in the stabilization of a deformed lumbar spine remains controversial Epidemiology Degenerative scoliosis can be differentiated into two major groups, i.e., primary degenerative scoliosis or de novo scoliosis (after skeletal maturity) and second- ary degeneration of adult idiopathic scoliosis or scoliosis of other etiology [1, 7]. Slow progression of degenerative scoliosis is common The prevalence of scoliosis in patients older than 50 years is about 6%, includ- ing patients with secondary degeneration of adult idiopathic scoliosis as well as patients with degenerative or de novo scoliosis [6, 7, 14, 17], and the average age of those seeking medical care with degenerative scoliosis is in the sixties. There is a potential for curve progression with an average of 3.3° a year ( Case Introduc- tion ). Degenerative scoliosis, which occurs on the basis of idiopathic scoliosis of less than 30°, usually does not tend to progress; however, curves greater than 50° have a tendency to progress an average of 1–2° a year. Nevertheless, for primary degenerative scoliosis, there is no scientific evi- dence which really documents the full complexity and extent of the natural his- tory. For instance, degenerative scoliosis occurs more frequently in male patients than adult idiopathic scoliosis, which is more frequent in females. There are several aggravating factors in patients with degenerative scoliosis, mostly duetotheadvancedageofpatients,whohaveseveralcomorbiditiessuchasdia- betes, heart disease, pulmonary disease, and osteoporosis, factors which play a significant role in the assessment and decision-making for treatment [3, 8, 11, 18, 25]. Spinal Deformities and Malformations Section 713 . depended on the type of vertebral anomaly, the location of the anomaly, and the age of the patient at diagnosis. Bradford DS, Heithoff KB, Cohen M (1991) Intraspinal abnormalities and congenital spine. failure of formation, failure of seg- mentation,ormixed. Examples of failure of forma- tion are hemivertebra and wedge vertebra, while unilateral unsegmented bars and block vertebra are examples of. spinal deformities the only way to rebalance the spine may be through a spinal column resection with shortening of the spinal column. This was described by Bradford and Tribus, and consists of