In the present study the morphologies of the VL, VM and VI and their relationships to additional heads of the QF were examined. The correlation between the additional QF heads and the patellar tendon was determined.
In recent years, increasing numbers of studies have examined morphological variations in human arteries, veins, nerves, muscles, tendons and ligaments 25,28–32. Morphological variations can have significant effects on nerve compression 33,34 or artery or vein compression 35, and an accessory hepatic artery or collateral circulation can facilitate surgical procedures 28. The most frequent morphological variations seem to be vascular and neural variations, followed by those of the musculoskeletal system 36.
While morphological variations can possibly be explained from an evolutionary perspective, it is first necessary to understand all possible variations. One example is the plantaris muscle, which was previously considered residual; but our research suggests that it can only develop in humans 37–39, like the fibularis tertius muscle 40. Complete understanding of all clinical and evolutionary aspects of the human body requires a full study of the morphological variability of the musculoskeletal and vascular organs.
Previous studies of the QF have tended to focus on the multilaminar nature of the vasti. The VL consists of three parts, referred to as superficial, intermediate and deep 41,42. The present study confirms this complexity of the VL. There is some evidence that the VL contracts before the VM in patients with patellofemoral pain syndrome; this has been proposed as a cause of knee pain 43. In addition, as each part is separately innervated, the muscle can be used for selective flap harvesting 41.
The VM comprises two parts, the VMO and VML 44. Our present findings confirm this, though our analysis was only superficial. Patellar maltracking and patellofemoral pain are treated by strengthening the oblique fibres of the VM to restore the balance between the VM and VL and assessing of the degree of dynamic supination and pronation of the foot 7,44.
Testut in 1884 first mentioned the morphological variability of QF, i.e. the lack of one or other part 45. He described a triceps crural (femoris) muscle comprising the RF, VM and VL 45. The same observation was made by LeDouble in 1897 2. Bonnechere et al. 8 reported four cases of triceps femoris attributable to the absence of VI and VM, or VI and VL.
In recent years there has been much controversy about the presence of a fifth head of the QF. The first reports of VL or VM duplication were by Le Double 2 and MacAlister 46, while Golland et al. noted a fifth head of the QF between the VL and VI in 1986 9. Holyoke 47 described a potential fifth head as an aberrant belly. Interestingly, Grob et al. 4 classified a fifth head of the QF as the tensor vastus intermedius (TVI) and created a five-fold classification of it, based primarily on the course of the aponeurosis tendon. A detailed description of this classification is given in Table 2 and Fig. 8.
This classification was confirmed in a study of 36 limbs of the South Indian population 10; however, the tendons showed little diversity. Another study found a TVI in only seven of 20 limbs 8; in addition, the VI and VM could not be differentiated, nor could the VI and VL.
Our recent research suggests that the Grob et al. classification 4 is not sufficient; however, an alternative based on proximal attachment has been proposed 7. This second classification has three main divisions with subtypes: Type I (subtypes A-B) includes a completely independent type TVI. Type IA originates from the upper level of the anterior surface of the greater trochanter, where it joins the intertrochanteric and gluteus medius ridge. The muscle belly runs laterally with respect to the VI. Type IB has the same proximal attachment as Type IA, but the muscle belly runs medial to the VI. Type II consists of three subtypes based on the source of the TVI (A-C); Type IIA from the VL, Type IIB from the VI, Type IIC from the gluteus minimus. In contrast, Type III has multiple heads; the number of subtypes depends on the number of additional muscles (six, seven or eight). Detailed descriptions and percentages of occurrence are presented in Table 1.
The incidence of the fifth head ranges from 29% to 100% 4,8,10,11. Some cases have a separate VL muscle belly (29% of dissected limbs) 9, or an additional muscle can be found between the VL and VI (36% of cases) 11. A TVI has also been found (35% of limbs) 8. Other studies have identified a TVI in all limbs 410; Olewnik et al. 7 found it in 64.1% of all cases.
The effects of presence or absence of the TVI are unknown, and is not clear whether it is a muscle that is still being developed, like the fibularis tertius 40 or plantaris 38,39,48,49, or a vestigial muscle like the palmaris longus 33.
The muscle in the musculotendinous junction was significantly thicker in Type IIIa than in all other types apart from Type IB. This could possibly allow people with Type IIIA and Type IB to transfer a much greater contraction force from the muscle belly to the tendon, thus reducing forces on the tendons in other parts of the QF. To confirm this, complex biomechanical tests will be needed.
The tendon of the VI was significantly longer in Types IIA and IIIA than in IB, IIB or IIIB. Generally, muscles with a wide and short tendon are characterized by the transfer of high contraction forces from the belly to the tendon 31. Future research must focus on comparing strengths among different types of extra heads.
The distal portion of the patella was wider in lower limbs without supplementary heads than in Type IA, but narrower than in Type IIIA. In general, the distal portion of the patella was narrower in specimens with supplementary heads than in those without (19.03 SD 3.18 mm vs. 20.58 SD 2.95 mm, p = 0.03817). It is noteworthy that the patellar ligament is wider when additional QF heads are present, indicating that the ligament can transfer much greater forces over the knee joint than when there are no such heads. Are people with extra heads therefore less likely to suffer from loss of QF muscle strength or knee injury?
The TVI muscle is needed for knee extension, and its action can be supported because it originates from the VI or VL. It is possible that it can improve rehabilitation after ACL reconstructions by hastening QF recovery. Interestingly, extension of the gluteus minimus tendon, from which the TVI originates, can also affect the hip joint.
This study has some limitations. First, the study population was recruited from specific populations of people who have lived the better part of their lives in the region around Lodz, Poland and around Madrid, Spain. In addition, the deep layer of the VM part was not examined extensively. Finally, our statistical results require further biomechanical research. Our findings in this article have revealed nothing about the effect of additional quadriceps femoris heads on the vasti muscles and patellar ligaments.