Psoas major muscle had been known for a long time as a hip flexor. The name Ilipsoas used in many literatures to express that both Iliacus and Psoas major have same action as a primary hip flexors. However, development in the knowledge of psoas major morphology and geometry changes this belief. Although the detailed spinal attachment of Psoas major is relatively complex; there is a common belief that psoas major essentially arises from transverse processes, body and intervertebral discs of all lumbar spines and body of 12th thoracic vertebrae (Standring 2004). McGill et al. (1988) considered Psoas major the largest muscle in Cross Sectional Area (CSA) at the level of lower vertebral spines.
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Anatomical and biomechanical studies in the 1980s and 1990s showed a lot of controversy about action of Psoas major. Moore (1983) reported that Psoas major is the dominant hip flexor, whereas (Woodburne and Burkel, 1988) concluded that Psoas major flexes lumbar spines and laterally rotates lumbar spines during pelvic fixation. Bogduck and Twomey (1987) indicated that Psoas major controls lumbar lordosis to support changes in lumbar load. Unfortunately, many of these studies were either built on incomplete understanding of fascicular anatomy of psoas major or used less accurate old imaging techniques in case of radiology in-vivo studies. For example, old MRI scanners emphasized cadaver overestimation in anterior/posterior shearing forces when compared with data gathered from dissection procedures.
The huge technical development in the last two decades such as using more accurate new Magnetic resonance Imaging (MRI) machines, pilot dissection techniques, new biomechanical devices and modeling techniques enhanced by computers hardware and software helped the researchers to gain better knowledge about psoas major morphology, geometry and functional anatomy. An example of studies used different models is (Sanlaguda and McGill 1995) which used MRI, modeling techniques in addition to pilot dissection procedures to develop knowledge about psoas major geometry.
Development of psoas major anatomical knowledge including bilateral variation and racial variations of psoas major are important to many health care professionals. Ramesh et al. (2008) reported the importance of discovering variations in geometry of psoas major for radiologists during interpretation of radiology images and for surgeons during surgical interventions. Psoas major plays role in function of both trunk and lower extremities (Hanson et al 1999). In addition, Knowledge of psoas major anatomy is essential to anaesthetist who works on a regional anaesthesia technique, the location of lumbar plexus within the psoas major gives importance of psoas major anatomy in anaesthiology science (Farny et al. 1994). Further more, Psoas major anatomy is important in physiotherapy practice, Barker et al. (2004) reported changes in CSA of psoas major in people who suffer from Unilateral Low Back Pain (LBP). Therefore, it is important for musculoskeletal rehabilitation practitioners to understand both anatomy of Psoas major during assessment and rehabilitation programmes of back and hip disorders.
The aim of this essay was to address and critique the accumulated anatomical knowledge of Psoas major in the available anatomical literatures. Various dissection procedures of psoas major either as a single muscle, in combination with other trunk muscles were discussed. Describing gross anatomy and fascicular anatomy of psoas major reviewed with an attempt to analyse study results as possible in this essay. Radiology imaging scaqnners such as Magnetic resonance imaging (MRI) used in Psoas major anatomical literatures discussed as essay space allowed. Attempts were exerted to find psoas major studies used ultrasound or x-ray machines without success. Discussing action of psoas major involved using biomechanical studies. Although this essay focused on discussing anatomical not biomechanical literatures of psoas major, discussing action of psoas major involved using few biomechanical literatures.
Dissection studies of psoas major
1.1 Fascicular anatomy
Bogduk et al. (1992) studied fascicular anatomy of the psoas major by dissecting three old males (above 60 years).Bogduk defined fascicle as “Psoas major portion shared a common discrete area of attachment in vertebral column and independent of each other in areas of attachment “. Bogduck exploratory cadaveric study presented valuable morphological and biomechanical data about both structure and action of Psoas major. For example, study showed that origin of psoas major fascicles arose from intervertebral disc and transverse process from level of T12-L1 to level of L4-L5, study has also the first which reported that each fascicle has its own tendon and that L4-L5 tendon forms central part of common psoas major tendon. Unlike many previous authors, Bogduk experiment found that equal length of Psoas major Fascicles not designed to flex or extend the lumbar spine but stabilize the lumbar spine during movement by exerting compression and shear forces on lumbar joints. Although this finding reported earlier by Crisco and Panjabi (1990), Bogduk study could explain the stabilization role of psoas major practically from anatomical (not biomechanical) view depending on detail description of independent action of psoas major fascicles. Bogduk study produced important data about Physiological Cross Sectional Area (PCSA) for each psoas major fascicle. These PCSA data in addition to line of action for each fascicle helped significantly to clarify that psoas major have multiple actions (fascicles attached to upper lumbar levels with the vertical line of action have mobilizing role and fascicles attached with lower lumbar levels with the horizontal line of action havestabilizing role).
However, methodological procedures described shortly in bogduk study, which was expected from an exploratory study. Bogduk Study used only three old male cadavers, which restricted its morphological and geometrical data (such as PCSA) to be old in active males only. It is not unusual to collect different morphological data from different age group or different gender in Psoas major, for example,Santaguida and McGill (1995) used different age groups to collect geometrical data of psoas major. In addition, cadaver status (whether fresh or preserved) and storage process not reported at all in the study.
1.2 Geometry of psoas major
Santaguida and McGill (1995) studied three dimensional geometric data of psoas major, Seven young male cadavers (five of them only used in measurement and two used to develop pilot dissection technique) used to collect morphological data, Unlike (Bogduk et al. 1992), Santaguida and McGill (1995) focused on measuring morphological features such as superficial muscle length, internal and external tendons length and angle of pinnation (i.e. angle betwen psoas major fibers and lumbar spines) rather than studying fascicular anatomy of psoas major. The important findings in Santaguida and McGill (1995) were that muscle fibers originating from higher lumbar levels displayed fewer steep angles (i.e. more vertically oriented than fibers originating at lower two lumbar levels) which confirms findings of Bogduk et al. (1992), Santaguida study found also that measurement of pinnation angel cannot be measured from a single plane. Measurement of pinnation angle is important to calculate PCSA. As same as Bogduk (1992), detailed methodological procedures not well reported in Santaguida and McGill (1995). Although the number of dissected cadavers bigger (five compared with only three in Bugduk study) and age of cadavers was younger but cadavers again were from same gender (male).
Racial differences in geometry of Psoas major discussed by Hansen et al. (1999). A large Number of male cadavers (fourty four) used to measure Psoas major length, width, anatomical CSA (ACSA) and circumference by using a measurement tool (digital caliper). Study introduced first (racial) data, Psoas major size was significantly larger in the black group than in the white group but no difference was noted related to muscle length and width between blacks and whites. Although dissection procedures described with more details when compared with procedures described in both Bogduk et al. (1992) and Santaguida and McGill (1995), dissection procedures in Hansen et al. (1999) still carried many (non well-explained) procedures. Certainly, study has not reported medical history of cadavers and activity level which well-known to affect CSA size. Furthermore, Authors have not explained why medical examiners were interested to exclude muscle bulk differences or anatomical variations in routine autopsies (specially if known that medical examiners not awarded about the study) . Sample selection bears a question too, cadaver’s age ranged from 14-25 years, which mean that some of the cadavers were still in development stage, which may be different according to the ethnic group. In addition, researchers tried to avoid bias by using medical examiner decision about race of cadavers. However, study has not clarified reliability of both measurement tools and testers (inter-tester and intra-tester reliability) of Psoas major geometry. Study also held in different centers without clarifying whether same dissection procedures followed exactly in all centres or not. Study also not clarified whether CSA measurement excluded tendinous fibers or not. All these notes indicated that results of psoas major CSA in the white group which is much lower than many other dissection studies such as Bogduk et al. (1992) and Santaguida and McGill (1995).
Ilayperuma and Nanayakkara (2008) studied gross anatomical characterization of the psoas major muscle by dissecting thirty four old Sri Lankan males and females. Study showed significant gender differences in ACSA at L4-L5 level, mean width and thickness (males got higher results). In addition, study showed that mean of psoas major ACSA (L4-L5 level) in Sri Lankan males was larger than the white group in Hansen et al (1999) but less than the black group in same study in L4-L5 level. However, study used same measurement tool in Hansen et al. (1999) and same comments on Hansen et al (1999) can be applied here too. Where the study held in a medical school it is necessary to clarify whether cadavers were fresh or preserved with any chemical embalming materials such as formaldehyde. Chemical embalming well-known to cause shrinking or distortion of cadavers, which alter muscle size, width, length and ACSA.
Bilateral variation of psoas major is rarely reported. Jelev et al. (2005) was the last available scientific report. Routine anatomical dissection of the retroperitoneal space of sixty seven old female cadaver showed that Femoral nerve was embedded between two bellies medial and lateral. The medial belly was the usual left psoas major but with wider origin wider ( transverse process and intervertebral discs of L1-L5 vertebrae and from the anterior (pelvic) surfaces of S1-S3 vertebrae).The lateral belly was unusual muscle situated behind and oblique (from dorso-medially to ventro-laterally) to the left psoas major found. This abnormal muscle originated from the left transverse process of L3 vertebra and from the intertransverse ligament between L3 and L4 vertebrae. Geometry inserted with a short tendon (1.8 cm) to the common tendon between the left psoas major and Iliacus. The right psoas major in same old female cadaver showed widened origin similar to the left psoas major. This study showed clearly chance to find variations (in this case bigger size of psoas major muscle), this variation is important to surgeons, anaesthiologists, radiologist and physiotherapists where the pattern of practice should be changed according to anomaly found within subjects. In physiotherapy practice, variation of psoas major muscle anatomy should change rehabilitation program duration and intensity.
Using MRI in Psoas Major Studies
MRI used extensively to measure geometry of psoas major in-vivo (Tracy et al. 1989, Parkkola et al. 1992, Tsuang et al. 1993, McGill et al. 1988, Santaguida at al. 1995, Wood et al. 1996, Baker et al. 2004). Table 2 shows Psoas major CSA data from MRI, CT and Dissection studies. However, different MRI data-acquisition techniques and data analysis process software were used, the difference between the measured spinal levels (where psoas major attached with) and difference between subjects used made a difference in the gathered data.
Copied from Gatton et al. (1999).
Copied from Gatton et al. (1999).
2.1 psoas major CSA in old and new MRI machines
. The MRI machines in the old studies (early 1990s and before) used outdated data acquisitions and software, which had large estimation errors (Mitsiopoulos et al. 1998). Accurate estimation of skeletal muscle mass (include psoas major) is important to make a correct comparison with data gathered from cadavers because MRI has a wide application and is a good way to gather in-vivo data from young people. The use of MRI as a reference standard is based on the assumption that measured CSA is equivalent to actual skeletal muscle CSA.
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Two types of muscle parameters can be considered during anatomical MRI scan of skeletal muscles, the first related to muscle tissue that includes interstitial adipose tissue (IAT) and the second, specifically related to adipose tissue-free skeletal muscle (ATFSM). In healthy young adults anatomical skeletal muscle is only slightly larger than ATFSM. However, IAT increases with increasing obesity and age. Some disease states, such as muscular dystrophy, are also accompanied by a relatively large mass of IAT. Recent MRI machines which use ATFSM measurement reference with more efficient hardware and upgraded software permit rapid whole body measurement in less than 30 minutes (Ross et al. 1996). Therefore, it is important to take notes about features of MRI machines used to collect data about various skeletal muscles (include psoas major) to get better idea about chance of estimation error. Almost all MRI studies of psoas major anatomy prior to 1995 used MRI machines follow IAT rather than ATFSM Scanning technique. Therefore, estimation errors are more plausible in studies prior done in the last century such as (Tracy et al. 1989, Parkkola et al. 1992, Tsuang et al. 1993) specially for old age people because the level of IAT higher in skeletal muscle than young people and technical capabilities of old MRI scanners.
New MRI scanners with more powerful hardware and software introduced new technology called Total Imaging Matrix (TIM). This technology allows patients to be scanned in most cases without their head passing into the magnet of the machine – reducing the claustrophobic element of the exam. The system is also much quieter than old MRI machines. In addition, TIM technology helped radiologists diagnose patients better as well. TIM images the body faster and can provide a clearer picture of what is being examined. Baker et al. (2004) used MRI scanner with TIM technology and scanning ATFSM (Magnetom Symphony, Siemens AG, Erlangen, Germany), CSA of Psoas major was significantly lower than 1990s studies (the lowest mean of CSA in Santaguida et al. (1995) was 421 mm in L1-L2 level, whereas in Baker et al. (2004) the biggest CSA reading was 197.7 in the right side symptomatic un-certianed level in the unilateral back pain patients. This huge difference in estimation of CSA between different MRI scanners with different technologies bears different hypothesis reasons. The first reason indicates that CSA of psoas major decreases significantly in Unilateral Back patients. The second reason indicates that new technologies of MRI imaging showed that in-vivo CSA of psoas major is much less than old CSA of psoas major gather by old MRI technologies. The third reason indicates that old studies (in table1) might measure other structures closely related to psoas major muscle such as iliacus, psoas minor or central tendon of psoas major specially in lower spinal levels rather than measuring psoas major only. Both the first and third reasons explain in the last sentence looks plausible, Dangaria and Naesh (1998) used MRI to compare CSA of Psoas major for both healthy volunteers and patients with Back patients with herniated discs and found that CSA decreased significantly in Back patients with herniated discs, although old MRI machines were used. Inaccurate description of the anatomical information found in (Parkkola et al. 1992, Tsuang et al. 1993, McGill et al., Wood et al. 1996, Baker et al. 2004), tested muscle was either described as iliacus (which may indicate measurement of both muscles CSA) or psoas (which may indicate combining psoas minor with psoas major).
All studies addressed in the last paragraph agreed in issuing two common trends related to subject characteristics. The first common character is that psoas major CSA is smaller in females, this decrease in female,s CSA explains why CSA in Parkkola et al. and Chaffin et al(table 2) are smaller than other studies where subjects of both studies were exclusively females. Lower than average CSA found also in Baker et al. (2004) and might derived from using female subjects in the patient or volunteer group. The second common character is the pattern of change in the psoas major CSA across spinal levels (increasing gradually from L2/L3 to L4/L5 and decrease at L5/S1).
2.2 Effect of MRI procedures on calculating Psoas major CSA
Effect of trunk position on ACSA of back muscles (include psoas major) in MRI studied by Jorgenson et al. (2003). Briefly, results showed that neutral trunk position during MRI imaging showed maximal ACSA and that ACSA at levels L4-L5 and L5-S1 intervertebral discs decreased by different percentage as the trunk moved from neutral about 45 flexions in the sagittal plane. Outcome of Jorgenson et al. (2003) study showed the importance of describing scan angle and trunk position during MRI scan to ensure getting the maximum CSA and minimize confusion during analysis of gathered psoas major CSA data. Reviewing table two studies showed that two studies only (Dangaria et al. 1997, Santaguida at al. 1995) taken care to describe the trunk and pelvic position during MRI imaging. Santaguida at al. (1995) reported that scan angle was cosine of 20 between the scan slice and line of action of psoas major muscle fibers at L4/L5 and L5/S1 levels, which reduced CSA at both levels, Dangaria et al. (1997) reported that all patients received special care to correct pelvic tilt before MRI imaging. However, the previous studies showed human position inside MRI scanner is vital in Estimating Psoas major CSA. Therefore, results of studies failed to introduce accurate description of subject position inside MRI scanners might was inaccurate.
This assignment aim is not to discuss biomechanical aspects of Psoas major muscle composition or action. However, using cadavers to calculate biomechanical forces helps to better understanding of muscle action. Yoshio et al. (2002) studied 25 osteoligamentous specimens with psoas major tendon to measure the flexion angle of the hip joint where psoas major tendon loses contact with the femoral head and pelvic surface. Further Ten osteoligamentous specimens used to measure tensile force and pressure exerted on the psoas major and/or bone tendon interface during contraction of psoas major. Tension force of psoas major tendon measured in different angles of hip flexion. Results suggested that psoas major muscle works as (i) erector of lumbar vertebral column and stabilizer of the femoral head in the acetabulum at 0° – 15° of hip flexion (ii)in 15°-45° of hip flexion, psoas major maint spine erection role and partially loses femoral head stabilization role (iii) 45°-60° of hip flexion psoas major works as flexor of lower extremity. Cadaver used in the study was old (above 70 years) from both genders, which restricted study sinding on the old age group. In addition, cadaver preparation procedures not described well in Yoshio et al. (2002). Study Procedures involved screwing lumbar spine and pelvis . First section of this essay showed that psoas major has stabilizing role in lumbar spine and fixation of the lumbar spine in Yoshio et al. (2002) lost one of the important roles of psoas major, which may have affected on calculation of psoas major action on the hip joint.
Knowledge about psoas major gained mainly by dissection and radiological images and specially MRI. Dissection helped to give knowledge about features and action of fascicles of psoas major,differences in geometry of psoas major (such as size, length and width) in different ethnic groups and genders. Bilateral variations and anomally of psoas major knowledge gained also from dissection although it was rare. ACSA and PCSA differences between different levels of lumbar spines were gained from cadaver dissection.
MRI was powerful in measuring geometry of psoas major in-vivo, studying psoas major geometry data for patients with specific disorders such as back pain perfomed mainly by MRI. Newer models of MRI offered quick and accurate imaging of human body. Biomechanical models offered valueable information about action for clarifying multiple actions of psoas major. Further histological studies is important to introduce new data about muscle fibers types of psoas major.
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