Clinical anatomy of the coccyx: A systematic review

REVIEW

Clinical Anatomy of the Coccyx: A SystematicReview{

JASON T.K. WOON AND MARK D. STRINGER*

Department of Anatomy, Otago School of Medical Sciences,University of Otago, Dunedin, New Zealand

The coccyx has been relatively neglected in anatomical research which is surprisinggiven the population prevalence of coccydynia and our inadequate understanding ofits etiology. This systematic review analyzes available information on the clinicalanatomy of the coccyx. A literature search using five electronic databases andstandard anatomy reference texts was conducted yielding 61 primary and 7 sec-ondary English-language sources. This was supplemented by a manual search ofselected historical foreign language articles. The coccygeal vertebrae, associatedjoints, ligaments and muscles, coccygeal movements, nerves, and blood supplywere analyzed in detail. Although themusculoskeletal aspects of the coccyx are rea-sonably well described, the precise anatomy of the coccygeal plexus and its distribu-tion, the function of the coccygeal body, and the anatomy of the sacrococcygealzygapophyseal joints are poorly documented. Further research into the anatomy ofthe coccyx may clarify the etiopathogenesis of coccydynia which remains uncertainin one-third of affected patients. Clin. Anat. 25:158–167, 2012. VVC 2011Wiley Periodicals, Inc.

Key words: coccygeal plexus; coccygeal body; coccygeal ligaments; ganglionimpar; coccydynia

INTRODUCTION

The coccyx is often regarded as a rather vestigialstructure that can be removed without apparentsequelae. Each year, more than 1,300 coccygecto-mies are performed in the USA (Agency for Health-care Research and Quality, 2011) and around 150within the public sector in England (Health and SocialCare Information Centre, 2011). The leading indica-tion for surgery is the treatment of coccydynia butcoccygectomy may also be used to treat rare coccyg-eal tumors such as sacrococcygeal teratoma, chor-doma, benign notochordal cell tumor, carcinoidtumor, and metastatic disease. The anatomy of thisregion is not only relevant to surgeons but also toanesthetists and pain specialists. This article reviewsthe clinical anatomy of the coccyx in an effort tohighlight deficiencies in our knowledge and stimulatefurther research into disorders such as coccydynia.

Literature Search

A literature search was conducted using the fol-lowing databases: EMBASE (Excerpta Medica 1947-

present); MEDLINE (1947-present); ISI Web ofKnowledge; PubMed; and the Cochrane Library.Figure 1 shows the search strategy. Papers con-taining original data were selected and secondaryreferences retrieved from bibliographies. Thesearch was largely restricted to English languagearticles but important foreign language papersand selected anatomy reference texts were alsoconsulted.

{A component of this review article formed the basis of a posterpresentation at ANZACA in Tasmania in December 2010 (WoonJT, Stringer MD, ‘‘What is the coccygeal plexus?’’).

*Correspondence to: M.D. Stringer, Department of Anatomy,Otago School of Medical Sciences, University of Otago, PO Box913, Dunedin, New Zealand.E-mail: mark.stringer@anatomy. otago.ac.nz

Received 3 February 2011; Revised 30 April 2011; Accepted 13May 2011

Published online 7 July 2011 in Wiley Online Library(wileyonlinelibrary.com). DOI 10.1002/ca.21216

VVC 2011 Wiley Periodicals, Inc.

Clinical Anatomy 25:158–167 (2012)

RESULTS

The following account is based on an analysis ofthe retrieved literature.

Osteology

The coccyx is named after the Greek word‘‘kokktx’’ due to its resemblance to the curved beakof the cuckoo (Sugar, 1995). It consists of three tofive coccygeal vertebrae, with four segments presentin about 70–80% of subjects (Le Double, 1912;Duncan, 1937). It accounts for only 0.4% of the dryweight of the vertebral column (Lowrance and Lat-imer, 1967). The first coccygeal segment is the larg-est, has transverse processes which may articulateor fuse with the sacrum, and is often separate fromthe progressively smaller caudal coccygeal vertebrae(Postacchini and Massobrio, 1983) (Fig. 2). Dorsally,the coccygeal cornua which are vestigial articularprocesses together with the posterior neural arch ofthe first coccygeal vertebra mark the inferior bound-ary of the sacral hiatus.

Fig. 1. Literature search strategy.

Fig. 2. Sagittal MRI T2-weighted image of the pel-vis of an adult male showing the coccygeal vertebrae.PS, pubic symphysis; B, bladder; P, prostate; R, rec-tum; SC, sacrococcygeal junction (courtesy of ProfessorTerry Doyle).

159Clinical Anatomy of the Coccyx

The length of the coccyx varies between individu-als but only about 10% of this variance is related tostature (Pelin et al., 2005). There are probable eth-nic variations. In a study of 200 adult Caucasianskeletons the mean vertical length of the coccyx was3 cm (Le Double, 1912), which compares with3.3 cm (range, 1.8–4.8 cm) in 50 Korean cadavers(Oh et al., 2004). In a study of Indian male cadav-ers, the curved distance between the apex of thesacral hiatus and tip of the coccyx was on average5.8 cm (range, 3.8–8.2 cm) (Aggarwal et al., 2009).

In most adults, the coccyx curves anteriorly andinferiorly. In about one-third of individuals it issharply curved or angulated with its tip pointingdirectly forwards, sometimes in association withanterior subluxation of a coccygeal vertebra (Postac-chini and Massobrio, 1983). Rarely, the tip of a nor-mal coccyx points slightly backwards in the sagittalplane, referred to as a retroverted coccyx (Kerimogluet al., 2007). It may also be deviated to a variableextent either side of the midline (Duncan, 1937).The angle between a line parallel to the vertical axisof the first and last coccygeal segment has beenused to quantify the degree of coccygeal curvaturebut there is a wide range of normal variation (Kimand Suk, 1999; Kerimoglu et al., 2007). Whether thecoccyx is less curved in women is uncertain since thedata are conflicting (Duncan, 1937; Postacchini andMassobrio, 1983).

Sacrococcygeal and Intercoccygeal Joints

Few anatomical studies have investigated the sac-roccygeal and intercoccygeal joints. Classically, thefirst coccygeal vertebra articulates with the sacrumvia a symphysial joint containing a thin interverte-bral disc of fibrocartilage (Saluja, 1988). This discmay have a cleft (Maigne et al., 1992) as is some-times seen in the pubic symphysis (Becker et al.,2010). However, in a study of 10 sacrococcygealjoints from elderly subjects, only five were symphy-ses; four were synovial and one was fused (Maigneet al., 1992). In this study, the first intercoccygealjoint was a symphysis in just two cases and synovialin the remainder. The frequency of sacrococcygealfusion in the general population is uncertain; it wasobserved in one-third of 120 adult radiographs inone study (Postacchini and Massobrio, 1983) but notrecorded in a single patient in another series of 112older adults undergoing dynamic magnetic reso-nance imaging (MRI) for defaecation disorders(Grassi et al., 2007). Studies of skeletal collectionssuggest that sacrococcygeal fusion is more likely inolder subjects but is not exclusively age related(Saluja, 1988).

The prevalence of fusion between coccygeal verte-brae in adults is variable but is not as simple as fourfused rudimentary vertebrae as stated in anatomytexts. In a radiographic study of 120 asymptomaticsubjects with a mean age of 38 years, 54% of thecoccyges consisted of two bone segments, 34% hadthree bone segments, 5% had four segments, and7% had a fully fused coccyx (Postacchini and Masso-brio, 1983).

On each side, the sacral cornu articulates with thecoccygeal cornu via a zygapophysial joint. This jointis barely described in anatomy reference texts whichis surprising given our inadequate understanding ofcoccydynia and the propensity for symptomatic facetjoint pathology elsewhere in the spine.

Ligamentous Attachments and FascialRelations

Numerous ligaments attach to the coccyx. The an-terior sacrococcygeal ligament attaches to the frontof the first and sometimes the second coccygeal ver-tebral bodies, blending superiorly with the termina-tion of the anterior longitudinal ligament (Standring,2008). The posterior sacrococcygeal ligament hasdeep and superficial components: the deep partattaches to the dorsal surface of the coccyx frominside the sacral canal merging with the posteriorlongitudinal ligament superiorly whilst the superficialpart descends from the inferior margin of the sacralhiatus to the coccyx sometimes closing the sacralcanal (Standring, 2008). The superficial part has amean length of about 3.4 cm (1.8–5.0) in malecadavers (Aggarwal et al., 2009). An intercornualligament runs between the sacral and coccygeal cor-nua on each side and a lateral sacrococcygeal liga-ment connects the transverse process of the firstcoccygeal vertebra to the inferolateral margin of thesacrum, completing the foramen for the fifth sacralnerve. The anococcygeal ligament, which is visibleon MRI scans (Rociu et al., 2000), runs between thetip of the coccyx and the posterior aspect of the mid-region of the external anal sphincter, just below thelevator ani raphe. Finally, the sacral attachments ofthe sacrotuberous and sacrospinous ligamentsextend on to the dorsolateral margins of the coccyx,spanning a distance in men of about 4.9 cm and 3.9cm, respectively, at their origin (Sagsoz et al., 2002;Hammer et al., 2009).

The filum terminale externum, sometimes errone-ously referred to as the coccygeal ligament, is thecontinuation of the spinal dura beyond the termina-tion of the dural sac at the second sacral vertebra.Distally, it fuses with the periosteum of the dorsalaspect of the coccyx or sacrum (Tubbs et al., 2005).A postanal pit may mark the site of its termination.In a histological study of 15 adult cadavers, the filumterminale externum was found to have a meanlength of 8 cm, mean width of 1 mm, and consistedlargely of fibrous tissue containing smooth muscle,blood vessels, and peripheral nerves with occasionalganglia (Tubbs et al., 2005). The latter may be rem-nants from second and third coccygeal nerves pres-ent in the fetus (Pearson and Sauter, 1971).

The presacral fascia lies anterior to the sacrumand coccyx and is a distinct layer visible both on MRIand at surgery. Laterally, it blends with the parietalpelvic fascia covering levator ani, ischiococcygeusand piriformis. Inferiorly, at the level of the anorectaljunction, it fuses with the posterior mesorectal fasciawhich encloses the peri-rectal fat (Garcia-Armengolet al., 2008; Standring, 2008). The presacral veinsare tethered to its posterior surface. Another layer of

160 Woon and Stringer

fascia, the rectosacral fascia, originates from thepresacral fascia at about S3 or 4 and descends ante-riorly to fuse with the posterior mesorectal fascia 3–5 cm above the anorectal junction; it anchors therectum to the sacral curvature (Garcia-Armengol etal., 2008) (Fig. 3).

Muscle Attachments

Both ischiococcygeus and the levator ani group ofmuscles are attached to the coccyx (Fig. 4). Ischio-coccygeus (coccygeus) is developmentally (Niikuraet al., 2010) and anatomically distinct from levatorani but is sometimes classified as part of the latter(Standring, 2008). From their wide pelvic origin themuscle fibers of levator ani converge posteriorly inthe midline to form a raphe which attaches to the tipof the coccyx. In many cadavers we have observedslips of muscle extending cranially from the ventralcoccygeal attachment of levator ani on either side ofthe coccyx anterior to the coccygeal attachment ofischiococcygeus to attach to the anterolateral aspectof the distal sacrum; these fibers may be remnantsof sacrococcygeus anterior, a muscle observed in fe-tal specimens (Niikura et al., 2010). Levator aniforms most of the pelvic floor thereby supporting thepelvic viscera and playing a critical role in continenceand childbirth (Barber et al., 2002). Ischiococcygeusand the sacrospinous ligament (phylogenetically adegenerate part of the muscle absent in mammalswith a mobile tail [Abitbol, 1988]) are attached tothe lateral margins of the fifth sacral vertebra andcoccyx. Gluteus maximus is attached to the lateralmargins of the dorsal aspect of the bone.

Coccygeal Movement and Function

A variable but small amount of flexion and exten-sion occur at the unfused sacrococcygeal joint. Flexionis produced by contraction of levator ani whilst exten-sion is mostly passive. A radiographic study in 47

healthy adults showed that the coccyx moved by amean of 98 in the sagittal plane when moving from thestanding position to sitting on a hard surface (Maigneand Tamalet, 1996) (Fig. 5). In almost one-third ofindividuals the coccyx extended by 5–158, in one-halfby less than 58, and in the remainder flexion between58 and 228 was observed. The authors concluded thata coccyx with a range of movement greater than 258was abnormally hypermobile. Asymptomatic subluxa-tion of less than one-quarter of the anteroposteriordimension of a coccygeal vertebra occurred in twocases when sitting (Maigne and Tamalet, 1996).

A more recent dynamic MRI study suggested thatthe coccyx is capable of even greater movement, atleast during defecation in patients with pelvic floordysfunction (Grassi et al., 2007). This study investi-gated coccygeal movement in defecating proctogramstaken in the supine position in 112 mostly femaleadults with no history of coccygeal pain. The coccyxmoved backwards through a mean arc of 12.58between contraction and relaxation of the pelvic floor;only one coccyx was immobile. There was no correla-tion between coccygeal movement and age.

When sitting, the ischial tuberosities bear most ofthe weight of the trunk but as the sitting personleans backwards, the coccyx is stated to form part ofa weight-bearing tripod (Foye and Buttaci, 2008).However, coccygectomy is not known to impair sit-ting balance and therefore this is probably of minorfunctional importance.

Coccygeal Nerves

The coccygeal nerve(s) have received little atten-tion which is surprising given the prevalence of coc-cydynia and our limited understanding of its etiology.Up to five coccygeal nerves are evident in humanembryos (O’Rahilly et al., 1990). Typically, all butthe first pair regress during human development,although a second coccygeal nerve may occasionallypersist (Pearson et al., 1966; Sato, 1980).

Anatomy texts usually describe the coccygealplexus as being formed by the ventral rami of the

Fig. 4. Anterior and posterior muscle attachmentsto the coccyx (Gray and Carter, 1858). This figure hasremained essentially unchanged in all subsequent edi-tions of Gray’s Anatomy.

Fig. 3. Retrorectal space after division of the recto-sacral fascia (necessary to mobilise the rectum). Arrows¼ rectosacral fascia; arrowheads ¼ presacral fascia;asterisks ¼ mesorectal fascia (courtesy of Dr. Juan Gar-cıa-Armengol, Head of Colorectal Surgery Department,Hospital General Universitario de Valencia, Spain).


fourth and fifth sacral nerves (S4 and S5) and thecoccygeal nerve (Co) (Romanes, 1986; Standring,2008). The coccygeal nerve emerges from the conusmedullaris and travels caudally within the caudaequina to exit the sacral hiatus with the S5 roots.Both nerves pass anterolaterally, S5 above the trans-verse process of the first coccygeal vertebra and Cobelow, S5 being joined by a small branch from S4; allthree nerves unite to form the coccygeal plexus onthe pelvic surface of ischiococcygeus. This plexusgives rise to anococcygeal nerves which are variablydescribed as (i) piercing the sacrotuberous ligament(Gray and Carter, 1858; Standring, 2008) or (ii)piercing ischiococcygeus and the anococcygeal liga-ment (Romanes, 1986; Moore et al., 2010), to supplythe skin of the anal triangle. Some authors also statethat the coccygeal plexus supplies (i) ischiococcygeusand part of levator ani (Romanes, 1986; Moore et al.,2010), (ii) ischiococcygeus and the posterior extrem-ity of the external anal sphincter (Sato, 1980), and(iii) the sacrococcygeal joint (Moore et al., 2010).However, modern dissection studies have shown that

ischiococcygeus is innervated by S3 and S4, with anoccasional contribution from S5 (Roshanravan et al.,2007) and levator ani is innervated by the nerves tolevator ani (predominantly S3 and S4 but also withan occasional contribution from S5) and branches ofthe pudendal nerve (Barber et al., 2002; Grigorescuet al., 2008).

The French anatomist, Leo Testut, depicted thecoccygeal plexus as a looping anastomosis betweenS4, S5, and Co (Testut, 1930) (Fig. 6). From thesuperior part of the loop branches are given off tothe inferior hypogastric plexus and the skin over thecoccyx. From the inferior part ‘‘internal’’ and‘‘external’’ branches pierce ischiococcygeus to supplythe skin over the coccyx and the fascia inferior togluteus maximus, respectively.

The dorsal ramus of Co in the embryo and fetusdoes not apparently divide into medial and lateralbranches like most other dorsal rami but insteadcontributes randomly to the dorsal sacral plexus,which supplies multifidus and the fascia and skinover the dorsal sacrum (Pearson et al., 1966) (Fig.7). However, one detailed dissection study of sixadult cadavers failed to identify a single cutaneousbranch arising from Co (Johnston, 1908).

Fig. 6. The coccygeal plexus (adapted from Testut1930). S4 and S5 are the fourth and fifth sacral nerves,respectively, and Co, the coccygeal nerve.

Fig. 5. Range of motion of the coccyx. The apex ofthe angle in the standing (bold line, B) and sitting(dotted line, C) positions is at the mid-sacrococcygealjoint. Redrawn with minor modifications from Maigneand Tamalet (1996).


Ganglion Impar

The sacral region of the sympathetic trunk usuallyconsists of four or five ganglia located medial or an-terior to the anterior sacral foramina (Potts, 1925)beneath the presacral fascia (Oh et al., 2004) (Fig.8). Occasionally, one or two coccygeal ganglia areseen (Oh et al., 2004). The first sacral sympatheticganglion is the largest and more caudal gangliabecome progressively smaller (Blaszczyk, 1981). Thesacral sympathetic chain is often asymmetric withabsent or fused ganglia and cross communicationsbetween each side are frequent (Blaszczyk, 1981).Each ganglion sends at least one grey ramus com-municans to its adjacent spinal nerve but up to 11

such branches from a single ganglion have beenreported (Potts, 1925). Sacral splanchnic nerves alsopass directly from the ganglia to the inferior hypo-gastric plexus (Potts, 1925).

The sympathetic chain converges caudally to forma solitary retroperitoneal structure, the ganglionimpar (or ganglion of Walther). This conveys sympa-thetic efferents to and nociceptive afferents from theperineum and terminal urogenital regions (Toshniwalet al., 2007). Ganglion impar blockade is used totreat intractable perineal pain of sympathetic originin patients with pelvic cancers (Oh et al., 2004).Contrary to some clinical descriptions (Toshniwal etal., 2007), the ganglion impar is located at a variablelevel between the sacrococcygeal joint and the tip ofthe coccyx. In one study of 50 Korean cadavers theganglion was most often located about one-third ofthe distance from the sacrococcygeal joint to the tipof the coccyx; the mean distance from the tip of thecoccyx (25mm) was directly related to the length ofthe coccyx (Oh et al., 2004). In 14% of cases theterminal ends of the sympathetic trunks united with-out forming a recognizable ganglion. In anotherstudy of 100 human fetuses the sympathetic trunkterminated at a classical ganglion impar anterior tothe first or second coccygeal vertebra (posterior tothe coccygeal body) in 62 specimens, but was apaired structure in 17, a unilateral ganglion in 11,and absent in 10 cases (Blaszczyk, 1981).

Blood Supply and Regional VascularAnatomy

The coccygeal region is principally supplied fromthe median and lateral sacral arteries, which run inthe presacral space. The median sacral arteryarises immediately above the posterior aspect ofthe aortic bifurcation, descends posterior to thesacral venous plexus usually to one or other side ofthe midline, and anastomoses with the lateral sac-ral arteries which send branches through the ante-rior sacral foramina (Esses et al., 1991; Tribus andBelanger, 2001; Guvencer et al., 2009). The me-dian sacral artery is about 2.3 mm wide at the coc-cyx (Tribus and Belanger, 2001; Guvencer et al.,2009). The coccygeal region is also supplied by asizeable coccygeal branch from the inferior glutealartery which is given off behind the sacrospinousligament and passes posteriorly to perforate thesacrotuberous ligament at multiple sites before ter-minating in gluteus maximus (Thompson et al.,1999).

The presacral venous plexus is formed by the me-dian and lateral sacral veins and their interconnec-tions; they drain into the left common iliac andinternal iliac veins (Baque et al., 2004). The plexuscommunicates with the internal vertebral venousplexus via intervertebral veins that pass through thesacral foramina (Baque et al., 2004).

Coccygeal Body

Microscopic arteriovenous anastomoses occurwithin all major organs of the body but are especially

Fig. 7. Diagram of the cutaneous branches of thedorsal sacral plexus in a human embryo. A large cuta-neous nerve from S3 (a) receives a contribution fromS1, S2, and possibly L5. Further branches arise fromthis cutaneous nerve (b) or directly from an intercon-necting loop (c) and travel caudally receiving a contri-bution from caudal levels (d). Sometimes, the anteriorramus of S5 may join the loops (e). Below S5, no con-sistent pattern is seen (Pearson et al., 1966). (Repro-duced with permission of John Wiley & Sons, Inc. fromPearson et al. (1966), Am J Anat 118:891-903.)


prominent in the palms and soles, fingers and toes,and ears (Sherman, 1963) where they are known asglomus bodies. The coccygeal body (or glomus coccy-geum) is an unusually large encapsulated arteriove-nous anastomosis found in the pericoccygeal softtissue adjacent to the terminal coccygeal vertebralsegment (Fig. 9). It was first discovered in 1859 byvon Luschka who later considered it ‘‘the seat of . . .coccygodynia’’ (Luschka, 1868). In a detailed morpho-logical study of 100 mostly adult specimens the coc-cygeal body measured on average 3.5 mm long by2 mm wide and 1.5 mm thick. In two-thirds of cases itwas located just anterior or posterior to the tip of thecoccyx, immediately caudal to the attachment of leva-tor ani; in the remainder it was either anterior (25%)or posterior (7%) to the terminal segments of the coc-cyx (Di Marino et al., 1975). In most cases Di Marinoet al. identified the coccygeal body as a single nodule,although others have shown that it is simply the larg-est of a network of glomus bodies found in the coccyg-eal region (Albrecht et al., 1994). Indeed, microscopicglomus bodies measuring between 50 and 450 lm indiameter have been found within coccygeal vertebrae,usually adjacent to bone but occasionally lying withinthe marrow fat (Albrecht et al., 1994). The coccygealbody is innervated by sympathetic and parasympa-thetic fibers from the nearby sympathetic trunk andpelvic splanchnic nerves (Conti et al., 2000).

Histologically, it consists of an afferent arteriole thatbranches into two to four coiled smaller arterioles.These blend to form a thicker walled arterial segment(a Sucquet-Hoyer canal) which in turn connectsdirectly to a vein (Bell et al., 1982; Santos et al.,2002). Up to four canals may be found within a coccyg-eal body and each canal is surrounded by several layersof modified smooth-muscle cells known as glomuscells. Mast cells have also been reported within the coc-cygeal body (Conti et al., 2000). The entire complex is

enveloped by a vascular connective tissue capsule con-taining nerves (Santos et al., 2002). Ultrastructurally,the endothelial cells lining the canals have an unusualcolumnar appearance (Sargon et al., 1998).

Glomus bodies in the extremities are consideredto control local blood flow and are important in tem-perature regulation but the role of the coccygealbody or bodies is unknown. Studies in animals haveshown no correlation between the size of the coccyg-eal body and length of the tail (Conti et al., 2000).Its role in the pathogenesis of coccydynia is ques-tionable (Albrecht et al., 1994). An important clinicalpoint is that peri- and intracoccygeal glomus bodiesshould not be mistaken for glomus tumours (Gatalicaet al., 1999; Gombos et al., 2008).

Lymphatic Drainage

The lymphatic drainage of the coccyx has notbeen studied in detail. One immunohistochemicalstudy using lymphatic endothelial cell markers foundno evidence of lymphatic vessels in the coccyx orassociated intervertebral discs, consistent with find-ings elsewhere in the spine (Kliskey et al., 2009).

Development of the Coccyx

The human embryo temporarily possesses a tailconsisting of up to ten coccygeal somites (Kunimoto,1918; Donovan and Pedersen, 2005). As embryonic

Fig. 9. Photomicrograph of the coccygeal bodylocated just inferior to the coccyx from a 90-year-oldfemale cadaver (Hematoxylin and Eosin).

Fig. 8. Sacral sympathetic trunk and ganglion impar(G). S4 ¼ 4th sacral nerve; S5 ¼ 5th sacral nerve; Co ¼coccygeal nerve. The ventral part of ischiococcygeus hasbeen removed to expose the coccygeal nerves.


development progresses, the most caudal coccygealvertebrae (which transiently number as many as six),their associated nerves, and the terminal segment ofthe developing spinal cord (which initially extends tothe coccyx) are resorbed (O’Rahilly et al., 1990). Ahuman tail is a rare congenital anomaly, usually con-sisting only of soft tissues but coccygeal vertebrae areoccasionally present (Donovan and Pedersen, 2005).These anomalies do not simply reflect a failure of cau-dal regression; their frequent association with spinaldysraphism and a tethered cord indicates a more com-plex developmental etiology (Alexiou et al., 2009).

Each coccygeal vertebra ossifies from a centrumwhich appears after infancy; the first coccygeal ver-tebra has additional secondary ossification centers inthe neural arch and transverse processes (Broome etal., 1998). Fusion of primary and secondary ossifica-tion centers may not be complete until about 30years of age (Broome et al., 1988).

Coccydynia

Coccydynia (or coccygodynia) has been the topicof recent clinical reviews (Nathan et al., 2010; Patijnet al., 2010) and a detailed account is beyond thescope of this article. Coccygeal pain, typically aggra-vated by sitting, standing, and/or walking, is aroundfour times more common in women than men (Kara-dimas et al., 2011). Most cases are considered to bedue to sacrococcygeal or intercoccygeal joint insta-bility which is frequently traumatic in origin. Traumamay be acute (e.g., a fall or childbirth) or chronicand repetitive, when obesity may be a predisposingfactor (Karadimas et al., 2011). Rare specific causesof coccydynia include infections and tumors. How-ever, one-third of cases are idiopathic (Gaspar et al.,2009; Bilgic et al., 2010; Trollegaard et al., 2010;Karadimas et al., 2011). Degenerative changes inthe sacrococcygeal intervertebral and intercoccygealjoints may explain some of these cases (Balain etal., 2006) but the zygapophyseal joint between thesacral and coccygeal cornua, and the coccygealplexus (with nerve entrapment) appear to have beenignored as potential pain generators.

Patients with coccydynia are assessed clinically andwith lateral sacral radiographs, which are probably moreinformative if performed standing and sitting (Maigneand Tamalet, 1996). Patients with a sharp ventral angu-lation of the coccyx are consideredmore at risk of devel-oping coccydynia (Postacchini et al., 1983), althoughlimited data are available. Selected patients can beinvestigated further using magnetic resonance imagingto detect occult instability (Mouhsine et al., 2006).

Numerous conservative treatments such as rubberrings, physiotherapy, nonsteroidal anti-inflammatorydrugs, and local neural blockade can be used(Nathan et al., 2010; Patijn et al., 2010) but manypatients come to coccygectomy. This completelyrelieves symptoms in 21–81% of patients (Cebesoyet al., 2007; Bilgic et al., 2010; Gaspar et al., 2009;Trollegaard et al., 2010) although the true efficacy ofcoccygectomy is likely to be less than this due topublication bias. Higher success rates have beenreported in some series in patients with a clear trau-

matic etiology or in those with demonstrable coccyg-eal instability (Maigne et al., 2000; Trollegaard etal., 2010).

The operative technique most commonly employedis attributed to Key (1937). In brief, a median longitu-dinal incision is made from the distal sacrum to the tipof the coccyx. The incision is deepened and the coccyxexposed by sharp dissection. The sacrococcygealintervertebral disc and the coccygeal ligaments are di-vided and the coccyx excised proximal to distal. Resid-ual ligaments are approximated with sutures. Thisprocedure undoubtedly damages all the ligamentsattaching to the coccyx including the anococcygeal lig-ament and most probably the coccygeal nerves butany functional consequences have not been investi-gated in detail. Subperiosteal resection may be anadvantage (Bilgic et al., 2010) but adequately pow-ered clinical trials have yet to be conducted.

Remarkably, reports on postoperative complica-tions are uncommon, apart from bacterial woundinfection (due to the close proximity of the anus)(Karadimas et al., 2011). Posterior rectal herniationfrom weakening of the pelvic floor is rare (Garcia etal., 1998; Miranda et al., 2009). There appear to beno reports of neurologic deficits but detailed postop-erative assessments are lacking.

SUMMARY

The coccyx is often perceived as a vestigial struc-ture, an opinion reinforced by the apparent paucity offunctional sequelae after coccygectomy. This mayaccount for the relatively limited information on itsanatomy. Yet it is the site of pain and disease and thereare remarkable gaps in our knowledge such as theprecise anatomy of the coccygeal plexus and its distri-bution; the function of the coccygeal body; and theanatomy and potential role of the sacrococcygeal zyga-pophyseal joints in coccydynia. This clinical review willhopefully stimulate further research in these areas.

ACKNOWLEDGMENTS

The authors wish to thank Robbie McPhee,Graphic Artist and Medical Illustrator for his assis-tance with Figures 5 and 6, and Mandy Fisher for herhelp in producing Figure 9.

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Clinical anatomy of the coccyx: A systematic review