Anatomy in practice: the sacrotuberous ligament.

ABSTRACT

It has been reported that via its attachment to the sacrotuberous ligament, the long head of the biceps femoris muscle is capable of influencing the motion or stability of the sacroiliac joint. However explanations of this mechanism focus primarily on these two structures and do not address the potential influence of surrounding tissues. A review of the anatomical literature was therefore undertaken to determine what muscles and ligaments are commonly associated with the sacrotuberous ligament. The morphology of this ligament in situ is presented and discussed so that readers may appreciate the clinical anatomy of this structure. Woodley SJ, Kennedy E, Mercer SR (2005). Anatomy in practice: the sacrotuberous ligament. New Zealand Journal of Physiotherapy 33(3) 91-94.

Keywords: sacrotuberous ligament, gluteus maximus, hamstring muscles

It has been proposed that biceps femoris plays an important role in influencing the stability and, or mobility of the sacroiliac joint (van Wingerden et al., 1997). This theory is based on an observed partial attachment of the long head of the biceps femoris muscle to the sacrotuberous ligament, which bridges the sacroiliac joint. It is hypothesised that force generated in the long head of biceps femoris affects sacrotuberous ligament tension, thereby dynamically influencing the sacroiliac joint. Evidence for this model comes from van Wingerden et al (1993; 1997) who concentrate on the morphological relationship of the long head of biceps femoris and the sacrotuberous ligament.

A review of anatomical texts revealed that many other muscles and ligaments are also associated with the sacrotuberous ligament, implying that the long head of biceps femoris does not necessarily act in isolation. The purpose of this paper is to present the anatomy of the sacrotuberous ligament in situ based on a survey of its descriptions in a variety of sources, and to discuss the possible implications of these associations.

Sacrotuberous Ligament

A variety of descriptions of the attachment sites of the sacrotuberous ligament were found in the literature. It is described by some as attaching the sacrum to the ischium (Levangie and Norkin, 2001), and by others as attaching the sacrum, ilium and coccyx to the ischial tuberosity (Agur and Dalley, 2005; Basmajian, 1982) (Figure 1). Another, more detailed, description emphasises its extensive proximal attachment which includes the posterior portion of the iliac crest, the lower three sacral vertebrae and the coccyx (Moore, 1985) (Figure 1). More specifically the attachment to the iliac crest is depicted to be at the posterior iliac spines (Romanes, 1964; Sobotta, 1909; Williams et al., 1995; Zuckerman, 1961), or between the posterior superior and posterior inferior iliac spines (Breathnach, 1965; Last, 1978; Palastanga et al., 1998). Resolving the differences between these descriptions is important if an anatomical description is to be used as the basis for clinical or biomechanical models. According to the first description, the entire sacrotuberous ligament passes between the sacrum and the ischium and so crosses the sacroiliac joint, which it could then influence. On the other hand in the remaining descriptions part of the ligament passes from the ischial tuberosity to the ilium and therefore does not span the sacroiliac joint. These latter descriptions suggest that only part of the sacrotuberous ligament may act across the sacroiliac joint.

[FIGURE 1 OMITTED]

Variation is also evident between descriptions of the ligament's sacral attachments. Basic descriptions include attachment to the dorsum and sides of the sacrum (Moore, 1985), the lateral margin of the sacrum (Sobotta, 1909; Zuckerman, 1961), or the back and sides of the sacrum below the auricular surface (Palastanga et al., 1998; Romanes, 1964). Slightly more detail is afforded by Breathnach (1965) who reports attachment to the lower part of the transverse tubercle of S3, the posterior aspect of the sacrum, and the transverse tubercles down to the coccyx. Again the choice of text would influence a reader's view regarding the specific proximal attachment site(s), and so will have some bearing on any hypotheses generated regarding the function of this ligament.

A coccygeal component of the sacrotuberous ligament is described in some texts (Last, 1978; Moore, 1985; Palastanga et al., 1998; Romanes, 1964; Sobotta, 1909; Williams et al., 1995; Zuckerman, 1961) but denied in others (Hollinshead, 1969). Its attachment is described as either the upper part (Last, 1978; Williams et al., 1995; Zuckerman, 1961), the side of the upper part (Palastanga et al., 1998; Romanes, 1964), the posterior surface (Sobotta, 1909), or the dorsum and sides of the coccyx (Moore, 1985). Knowledge of the precise coccygeal insertion sites allows consideration of the full extent of the superomedial attachment of the sacrotuberous ligament. This means that if significant force does pass through the sacrotuberous ligament, it could also impact on structures associated with the coccyx, such as the pelvic floor muscles.

From its broad superomedial attachments the fibres of the sacrotuberous ligament converge as they pass downwards, laterally and slightly anteriorly towards the ischial tuberosity (Figure 1). As they approach the ischial tuberosity the fibres twist on themselves and diverge again before attaching to the medial margin of the tuberosity, and to the lower margin of the ramus of the ischium (Figure 2) (Last, 1978; Romanes, 1964; Williams et al., 1995; Zuckerman, 1961) and pubis (Last 1978; Palastanga et al., 1998; Sobotta 1909). The fibres to the ischial ramus, which form a sickle shaped extension, are known as the falciform ligament (Romanes, 1964) (Figure 2).

[FIGURE 2 OMITTED]

Morphological Associations

The sacrotuberous ligament is intimately associated with a variety of ligaments and muscles. Each of these structures may interact with the ligament differently and affect its function.

Textbooks have described the sacrotuberous ligament as partly blending (Hollinshead, 1982; Sobotta, 1909; Williams et al., 1995), blending (Hollinshead, 1969; Romanes, 1964)) or connecting (Sobotta, 1909) with the dorsal sacroiliac ligaments (Figure 1). The ligament is also portrayed as being in contact and partially fused with the sacrospinous ligament (Breathnach, 1965). Other authors suggest that these two ligaments blend (Hollinshead, 1969; Williams et al., 1995) or, where they intersect the sacrotuberous ligament is adherent to the sacrospinous ligament (Sobotta, 1909; Testut, 1899) (Figure 1). The close associations with both the dorsal sacroiliac ligament and the sacrospinous ligament may affect the amount and direction of force transmission within the sacrotuberous ligament. Forces passing through the sacrotuberous ligament could be augmented, attenuated or redirected via the sacrospinous or dorsal sacroiliac ligaments.

Review of the anatomical literature also revealed that the muscle primarily associated with the sacrotuberous ligament is gluteus maximus (Figure 3), with the posterior surface of the ligament providing an extensive area of insertion for this muscle (Charpy and Nicolas, 1912; Last, 1978; Moore, 1985; Palastanga et al., 1998; Shellshear and McIntosh, 1949; Testut, 1899; Williams et al., 1995). This ligamentous origin therefore indirectly attaches the gluteus maximus muscle to the sacrum (Breathnach, 1998). So the portion of the sacrotuberous ligament that crosses the sacroiliac joint - which is commonly associated with biceps femoris, is also intimately associated with gluteus maximus. Therefore tension developed in this large muscle could influence the sacrotuberous ligament, just as is postulated for the biceps femoris muscle. Similarly the piriformis (Hollinshead, 1969, Moore, 1985; Romanes, 1964; Testut, 1899; Williams et al., 1995), obturator internus (Testut, 1899), semitendinosus and semimembranosus muscles (Bergman et al, 1984; Le Double, 1884) have also been described as attaching to the sacrotuberous ligament (Figure 3 and 4). Consequently all these muscles have the potential to influence force transmission through the sacrotuberous ligament.

[FIGURES 3-4 OMITTED]

The association between the long head of biceps femoris and the sacrotuberous ligament is well established, but it should be noted that this association is only partial. The sacrotuberous ligament is considered to be the phylogenetically degenerated tendon of origin of the long head of biceps femoris (Hollinshead, 1969; Last, 1978; Palastanga et al., 1998), but this does not mean that these two structures are entirely continuous. In addition to attaching to the lower part of the sacrotuberous ligament, the tendon also attaches proximally to an inferomedial impression on the upper area of the ischial tuberosity (Williams et al., 1995) (Figure 4). It has been reported that only the superficial fibres of the lower part of the sacrotuberous ligament are continuous with the tendon of biceps femoris (Palastanga et al., 1998; Williams et al., 1995; Romanes 1964), while the greater part of the tendon attaches directly into the ischial tuberosity. Therefore the full tension generated by the muscle cannot be expected to pass directly to the sacrotuberous ligament.

Finally, the sacrotuberous ligament is pierced by the coccygeal branches of the inferior gluteal artery, and the perforating cutaneous nerve and filaments of the coccygeal plexus (Williams et al., 1995). The close relationship with blood vessels and nerves suggests that this ligament may not be subject to high loads, as heavily loaded structures are not commonly associated with the passage of neurovascular bundles.

CONCLUSIONS

Review of anatomical texts demonstrates that discussion of the structure and function of the sacrotuberous ligament cannot occur in isolation, and emphasises that the full detailed anatomy must be considered when developing clinical or biomechanical models. The long head of biceps femoris may influence load transmission through the sacrotuberous ligament, but so too may the action of gluteus maximus, piriformis, obturator internus, and perhaps the other hamstring muscles. Further modification of force transmission may also occur via its relationship with the dorsal sacroiliac and sacrospinous ligaments.

ACKNOWLEDGEMENTS

Our thanks to Mrs Shannon O'Neill, Mr Brynley Crosado and Mr Russell Barnett for preparing the specimens which are used to illustrate this paper.

Key Points

* The sacrotuberous ligament has broad attachments to the ilium, sacrum, coccyx and ischium.

* In addition to part of the long head of biceps femoris, a number of ligamentous and muscular structures are intimately associated with the sacrotuberous ligament.

REFERENCES

Agur AMR and Dalley AF (1991): Grant's atlas of anatomy. Baltimore: Lippincott Williams & Wilkins, pp 299.

Basmajian JV (1982): Primary anatomy (8th ed.) Baltimore: Williams & Wilkins, pp 64.

Bergman RA, Thompson SA and Afifi AK (1984): Catalog of human variation. Munich: Urban & Schwarzenberg, pp 54-55.

Breathnach AS (1965): Frazer's anatomy of the human skeleton. London: J & A Churchill, pp 40-43, 111-112.

Charpy A and Nicolas A (1912): Traite d'anatomie humaine. Paris: Masson et Cie, pp 238-240.

Hollinshead WH (1982): Anatomy for surgeons: the back and limbs. London: Harper & Row, pp 628.

Hollinshead WH (1969): Anatomy for surgeons: the back and limbs. London: Harper & Row, pp 643.

Last RJ (1978): Anatomy. Regional and applied. Edinburgh: Churchill Livingstone, pp 355.

Le Double JL (1884): Les anomalies musculaires chez l'homme. Paris: G Masson, pp 1-844.

Levangie PK and Norkin CC (2001): Joint structure and function. A comprehensive analysis. Philadelphia: FA Davis Co, pp 147-148.

Moore KL (1985): Clinically orientated anatomy. London: Williams & Wilkins, pp 442- 443.

Palastanga N, Field D and Soames R (1998): Anatomy and human movement. Structure and function. Oxford: Butterworth & Heinemann, pp 416.

Romanes GJ (1964): Cunningham's textbook of anatomy. London: Oxford University Press, pp 243 -244.

Shellshear JL and Macintosh NWG (1949): Surveys of anatomical fields. Sydney: Grahame Book Company, pp 86.

Sobotta J (1909): Atlas and textbook of human anatomy. London. WB Saunders, pp 129.

Testut L (1899) Traite d'anatomie humaine. Vol 1. Paris: Octave Doin, pp 555-557.

van Wingerden J-P, Vleeming A, Snijders CJ and Stoeckart R (1993): A functional-anatomical approach to the spine-pelvis mechanism: interaction between the biceps femoris muscle and the sacrotuberous ligament. European Spine Journal 2: 140-144.

van Wingerden J-P. Vleeming A, Kleinrensink G-J and Stoeckart R (1997): The role of the hamstrings in pelvic and spinal function. In: Vleeming A, Mooney V, Snijders CJ, Dorman TA, Stoeckart R (Eds): Movement, stability and low back pain. The essential role of the pelvis. Edinburgh: Churchill Livingstone, pp 207-210.

Williams PL, Bannister LH, Berry MM, Collins P, Dyson M, Dussek JE and Ferguson MWJ (1995): Gray's anatomy. The anatomical basis of medicine and surgery (38th ed.) Edinburgh: Churchill Livingstone, pp 677, 879.

Zuckerman S (1961): A new system of anatomy. London: Oxford University Press, pp 341-342.

ADDRESS FOR CORRESPONDENCE

Dr SR Mercer, School of Biomedical Sciences, The University of Queensland, Brisbane, Q4072, Australia.

SJ Woodley, MSc, BPhty

E Kennedy, BPhty

PhD students

Department of Anatomy and Structural Biology

University of Otago 

Dunedin, New Zealand

SR Mercer, PhD FNZCP

Associate Professor

School of Biomedical Sciences

The University of Queensland

Brisbane, Australia.