Gemelli Muscles
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| Gemelli Muscles | |
|---|---|
| Muscle Type | |
| Origin |
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| Insertion |
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| Action | Externally rotates and stabilises the hip |
| Synergists | |
| Antagonists | |
| Spinal innervation | |
| Peripheral Innervation | |
| Vasculature | |
The inferior gemellus is the “twin” to the superior gemellus, located just below the obturator internus tendon. It is another small lateral rotator in the deep gluteal region. Along with the superior gemellus and obturator internus, it constitutes the triceps coxae. The inferior gemellus laterally rotates the extended thigh, abducts the flexed thigh, and aids in stabilizing the hip joint by steadying the femoral head. Though minor in size, it contributes to the precision of hip movements and stability.
The superior gemellus is a small muscle in the deep gluteal region, named “gemellus” (Latin for twin) because it is paired with the inferior gemellus, positioned above and below the obturator internus tendon. The superior gemellus lies just inferior to the piriformis and serves as an accessory muscle to the obturator internus. It helps laterally rotate the extended thigh and abduct the flexed thigh at the hip. It also acts to stabilize the femoral head. Though small, it is important in the coordinated action of the lateral rotator group (triceps coxae formation with obturator internus and inferior gemellus).
Structure
Inferior gemellus is a small, rectangular muscle in the deep gluteal region. It mirrors the superior gemellus in form and function, but lies inferior to the obturator internus tendon. It is often inseparable from surrounding fibers in dissection and essentially acts as a supplemental part of the obturator internus mechanism.
Superior gemellus is a small triangular muscle in the deep gluteal region. Along with the inferior gemellus and obturator internus, it forms part of the so-called triceps coxae (three-headed muscle) that spans the area between the piriformis above and quadratus femoris below. It has a relatively short muscle belly.
Origin
Inferior gemellus originates from the upper aspect of the ischial tuberosity, just below the groove for the obturator internus tendon (which exits the lesser sciatic foramen). This origin is adjacent to where the hamstrings originate (but on the lateral facet of the tuberosity).
Superior gemellus arises from the ischial spine, the pointed projection of the ischium that lies between the greater and lesser sciatic notches. This is a bony origin just above the lesser sciatic foramen. The ischial spine is also where the sacrospinous ligament attaches; the superior gemellus originates from the outer (gluteal) surface of the spine.
Insertion
Inferior gemellus inserts onto the trochanteric fossa on the medial side of the greater trochanter by joining the inferior margin of the obturator internus tendon. Thus, like the superior gemellus, it blends with obturator internus, but on the lower border of the tendon. The common tendon then attaches to the femur.
Superior gemellus inserts into the trochanteric fossa (medial surface of greater trochanter) via the tendon of the obturator internus. Essentially, the superior gemellus muscle joins the upper border of the obturator internus tendon. Thus, it shares the insertion with obturator internus and inferior gemellus. This arrangement means the gemelli reinforce and help direct the force of obturator internus.
Action
The inferior gemellus externally rotates the thigh when the hip is extended, working as part of the lateral rotator group. When the hip is flexed, it assists in abducting the thigh (pulling the femur outward). It also contributes to hip stability by preventing excessive internal rotation and maintaining congruence of the femoral head in the socket during movements. Essentially, its actions are identical to the superior gemellus, just positioned slightly lower.
The superior gemellus, working with obturator internus, externally rotates the thigh when the hip is extended. When the thigh is flexed, it helps abduct the femur (because the line of pull changes relative to the joint). It also contributes to hip joint stability by steadying the femoral head in the acetabulum (especially in conjunction with the obturator internus tendon acting as a dynamic ligament). Its role is minor in terms of force (due to small size) but significant in fine-tuning movements and maintaining congruency of the hip.
Synergists
It works together with the obturator internus and superior gemellus as the triceps coxae to rotate the hip laterally. It also synergizes with quadratus femoris (for lateral rotation, especially in the lower ranges of hip motion) and piriformis (for abduction of the flexed hip). All the short external rotators (piriformis, gemelli, obturators, quadratus) often co-contract to stabilize the hip or produce rotation. When the thigh is flexed and these muscles abduct, they assist the gluteus medius/minimus in that movement.
The superior gemellus works in synergy with the obturator internus (whose tendon it attaches to) and the inferior gemellus as a functional unit. They all share the same actions. It also synergizes with the piriformis (above) and quadratus femoris (below) in lateral rotation. For abduction of the flexed hip, it works with piriformis and obturator internus. All lateral rotators together stabilize the hip during weight-bearing, contracting to hold the head of femur secure. The superior gemellus is often activated simultaneously with obturator internus during movements, effectively acting as one muscle.
Antagonists
The internal rotators of the hip (gluteus medius/minimus anterior fibers, TFL) oppose its lateral rotation. Also, in the flexed hip when it contributes to abduction, the adductors oppose that. But since inferior gemellus has no unique action apart from the group, its antagonists are essentially the same as for obturator internus or superior gemellus. One could note that gravity and body weight forcing internal rotation/adduction of the hip are effectively countered by gemelli; if gemelli are weak, you’d see more internal rotation/pronation during stance (so their antagonists are also the internal rotation forces).
Anatoginists of the superior gemellus are the gluteus medius and minimus (anterior fibers) and tensor fasciae latae antagonize its lateral rotation by internally rotating the femur. Also, when the hip is flexed and gemellus assists in abduction, the hip adductors oppose that motion. However, since in extension gemellus does no abduction, the primary antagonistic consideration is against internal rotators during extension. Practically, during gait, the internal rotators turn the pelvis forward on the fixed femur, which gemellus (with others) will eccentrically control.
Spinal Innervation
Inferior Gemellus: L4, L5, S1 (via nerve to quadratus femoris).
Superior Gemellus: L5, S1, S2 through the nerve to obturator internus.
Peripheral Innervation
Inferior gemellus: Nerve to Quadratus Femoris (from the sacral plexus) innervates the inferior gemellus. This nerve, after innervating quadratus femoris on its deep surface, gives a twig to inferior gemellus. It runs in front of these muscles near the hip joint. Thus, damage to this nerve (for example, in deep posterior hip dislocations or surgical misadventures in the sciatic notch) would knock out quadratus femoris and inferior gemellus together.
Superior gemellus: The Nerve to Obturator Internus innervates the superior gemellus (it gives a branch to gemellus as it exits the pelvis). This nerve originates from the sacral plexus (anterior division of L5–S2). It passes through the greater sciatic foramen below piriformis, innervates the superior gemellus, then re-enters the pelvis via the lesser sciatic foramen to supply obturator internus. Injury to this nerve (rare except in deep pelvic surgeries or extreme sciatic notch trauma) would weaken both obturator internus and superior gemellus.
Vasculature
Inferior gemellus: The inferior gluteal artery provides blood via small muscular branches. The inferior gemellus also lies near the cruciate anastomosis of the hip (where inferior gluteal and medial circumflex femoral arteries join), so it has a good collateral blood supply. Given its small size, it doesn’t have a dedicated named artery beyond these contributions.
Superior gemellus: Blood supply is from the inferior gluteal artery via small muscular branches, and possibly contributions from the internal pudendal artery (as it passes out of the greater sciatic foramen near the ischial spine). The superior gemellus is small, so its blood demand is modest. It sits in the watershed area of inferior gluteal and internal pudendal circulation. No significant vascular issues are associated with it.
Clinical Relevance
Inferior Gemellus
Relation to Sciatic Nerve: The sciatic nerve typically runs above the inferior gemellus (it passes below piriformis and on top of the obturator internus & gemelli). In cases of sciatic nerve entrapment by fibrous bands (deep gluteal syndrome), sometimes the band can be between the inferior gemellus and the sciatic nerve. Releasing such bands or even partially releasing the gemellus could be part of surgical treatment if identified. However, piriformis remains the more common culprit.
Ischiofemoral Impingement (IFI): While quadratus femoris is the hallmark muscle affected in IFI, in severe narrowing, the inferior gemellus (just above quadratus) could theoretically also be compressed. Patients might have tenderness in the deep buttock. On MRI, IFI is noted by quadratus femoris edema; if very extensive, it could reach up to inferior gemellus region (though usually gemellus superior/inferior are too small to show distinct changes). The management remains addressing the bony impingement.
Exercise and Stretching: Inferior gemellus is engaged during exercises that target the deep rotators, such as resisted external rotation (clamshell exercise). When stretching the external rotators (e.g., figure-4 stretch or pigeon pose), inferior gemellus is stretched along with the others. If very tight, one might feel a deep stretch in the buttock. Some advanced massage techniques aim for the “gemellus” region by applying pressure just above the sit bone for inferior gemellus (though it’s deep and often inaccessible without deep pressure).
Surgery: In posterior approaches to the hip (like the Kocher-Langenbeck for acetabular fractures), sometimes surgeons must detach the short external rotators including inferior gemellus to get adequate exposure, then repair them. Not repairing inferior gemellus (and its cohorts) can slightly reduce external rotation strength and joint stability; surgeons typically try to reattach the conjoined tendon of gemellus/internus if cut.
Anatomical Variants: Occasionally, the gemelli may be fused with the obturator internus or even absent. Absence of inferior gemellus is rare but has been reported; obturator internus can compensate for the minor loss. Conversely, an accessory gemellus might exist. Such variations usually have no clinical effect unless encountered during surgery or imaging.
Nerve Block Effects: When a sciatic nerve block is done, it doesn’t affect gemelli directly (they’re innervated by separate nerves). However, in pain conditions, a Pudendal nerve block or posterior femoral cutaneous nerve block might relieve some deep gluteal pain that could be attributed to structures around inferior gemellus. But these are not direct treatments for gemellus issues – more for pain that might radiate from the area.
Evaluation: Like superior gemellus, you can’t isolate inferior gemellus in a clinical exam. But if you note weakness in external rotation and imaging or EMG shows quadratus femoris and inferior gemellus involvement (nerve to quadratus femoris lesion), then inferior gemellus is implicated. Largely, inferior gemellus issues ride along with quadratus femoris issues (since same nerve). So, an inability to laterally rotate the hip strongly with the hip extended might hint at such a lesion, but again, other rotators would compensate to a degree.
Pelvic Fractures: In fractures involving the ischial tuberosity or posterior acetabulum, the inferior gemellus might be damaged or entrapped in callus. If a patient post-healing has persistent deep buttock pain, entrapment of these deep rotators in scar or malunion might be considered. Releasing them surgically could relieve pain but is rarely pursued unless major functional deficit.
Superior Gemellus
Deep Gluteal Syndrome: While piriformis is often blamed in sciatic nerve entrapment, a tight or hypertrophied superior gemellus (with obturator internus) could also contribute to deep gluteal syndrome. In cyclists or skaters who externally rotate a lot, these muscles can become overused. Symptoms would be similar to piriformis syndrome (deep buttock pain). Stretching and soft tissue therapy addressing all external rotators (not just piriformis) can be beneficial.
Injury: Direct injury to superior gemellus specifically is uncommon. It could potentially be torn in an extreme internal rotation injury, but usually the force would affect the obturator internus or bone first.
Surgical Consideration: In sciatic nerve release surgeries or pelvic fracture repairs, identifying the “tricipital” region (gemellus superior, obturator internus, gemellus inferior) is important since the pudendal nerve and vessels exit below the superior gemellus, and the sciatic nerve lies above it. Surgeons use the ischial spine (where superior gemellus originates) as a landmark for the sacrospinous ligament and nearby neurovascular structures.
Trendelenburg? Superior gemellus is too small to significantly impact pelvic tilt if weak; that's handled by medius/minimus. But if superior gemellus (and co.) are weak or inhibited, one might see slight impairment in external rotation strength or stability (manifesting perhaps as slight toe-out gait to avoid needing internal rotation control). However, such deficits are subtle and typically masked by other muscles.
Entrapment of Nerve to Obturator Internus: Very rarely, a lesion compressing the nerve to obturator internus (like a mass in the ischioanal fossa region or pelvic outlet) could denervate superior gemellus and obturator internus. Clinically, this might cause vague lateral rotator weakness and possibly pelvic floor issues (since the nerve runs near pelvic floor). This scenario is more academic than common.
Evaluation: On physical exam, you cannot isolate superior gemellus function. However, if you resist external rotation or have the patient externally rotate from a flexed position, you are testing the group including gemellus. Pain with that motion, if not piriformis or quadratus (via imaging), might implicate obturator internus/gemelli. For instance, Beatty’s maneuver (FAIR test for piriformis) might also stress the obturator internus/gemelli group. If EMG were done, superior gemellus would be evaluated through the obturator internus nerve stimulation.
Synovial Cysts: Occasionally, paralabral cysts from the hip joint can extend into the ischiofemoral space and compress structures like the obturator internus/gemellus area. This might present as buttock pain that is exacerbated by rotation. MRI would show the cyst. Treatment is cyst aspiration or hip labrum treatment. This isn’t directly a superior gemellus pathology, but involves its neighborhood.
