Transfemoral Prosthetics

Section 1: The Vascular Reality, the Residual Limb, and the Two Foundational Sockets

Bottom line: ~85% of major lower-limb amputations are vascular (most often diabetic); energy cost per unit distance rises to ~55-65% (traumatic) or ~100-120% (vascular) above baseline at TF (vs 10-25% / ~40% at TT); ideal residual limb is conical with ≥5-7 cm distal to the lesser trochanter and a critical adductor myodesis; the two foundational sockets are the Radcliffe quadrilateral (narrow AP, wide ML, ischium ON brim, femur abducted) and the modern ischial containment / MAS (narrow ML, wider AP, ischium INSIDE, femur in anatomic adduction with optimized gluteus medius leverage).

Transfemoral (above-knee) amputation sacrifices the knee joint, eliminating the body’s most important energy-conserving mechanism and raising oxygen cost per unit distance to 55-65% (traumatic) or 100-120% (vascular) above baseline, vs 10-25% and about 40% at the transtibial level. The prescription involves four interdependent elements: socket, knee unit, suspension, and foot-ankle system.

Approximately 85% of major lower-limb amputations result from peripheral vascular disease, most commonly diabetes; the single most-tested transfemoral fact. The typical vascular TF amputee is 60-80 years old with cardiovascular comorbidities, an at-risk contralateral limb (~50% 5-year amputation risk), neuropathic skin, unstable limb volume from edema and dialysis, reduced dexterity, and possible diabetic retinopathy or cognitive impairment. Most are K1-K2 at best; many are K0 and never become functional users. The 100-120% above-baseline energy cost often exceeds cardiovascular capacity, driving wheelchair dependence and underscoring why knee preservation via transtibial amputation is so critical. Component selection must account for reduced dexterity (pin-lock often preferred over suction), impaired sensation (frequent skin checks), and volume instability (adjustable sockets or elevated vacuum). The contralateral limb requires aggressive prevention via foot exams, fitted footwear, glycemic and cardiovascular control.

Functional outcome is directly proportional to residual limb length, muscle strength, and soft tissue quality. Ideal shape is conical; the femur should be preserved ≥5-7 cm distal to the lesser trochanter, ideally 50-70% of original length. Myodesis (muscle and fascia anchored to bone through drilled channels) is preferred over myoplasty (opposing muscles sutured to each other over the cut bone end, which can raise a bursa and weaken the pull). Adductor myodesis is critical: without it, unopposed hip abductors create an abduction contracture and lateral socket instability. The exception matters in this population: bone fixation needs a limb that can heal it, so in a severely dysvascular or frail elderly limb the surgeon may take the muscle-to-muscle repair instead, and an absent myodesis there is a perfusion decision rather than a technical failure. Hip flexion contracture >20 degrees is very difficult to accommodate; abduction contracture is even more problematic for ML stability.

The Thomas test assesses hip flexion contracture: patient supine, contralateral hip flexed to flatten lordosis, and the angle the amputated limb rises is the contracture. 0-10 degrees: easily accommodated. 10-20 degrees: accommodated by matching socket flexion, with moderate energy cost and a shortened sound-side step (restricted prosthetic-side hip extension limits the opposite step). >20 degrees: dramatically impairs function; a 35-degree hip flexion contracture raises walking energy consumption by about 60%. Prevention via prone lying, avoiding prolonged sitting, no pillow under the limb, and hip extension stretching is far more effective than later accommodation.

The quadrilateral socket (Radcliffe, UC, 1950s) has a rectangular cross-section with ML wider than AP. The ischial tuberosity sits ON the posterior brim (the “ischial seat”) and does not enter the socket; the flat posterior shelf bears weight. The anterior wall rises to the inguinal ligament with a Scarpa’s bulge for the femoral triangle. The lateral wall provides minimal femoral containment; the greater trochanter is not captured. Weight-bearing is across the ischial tuberosity, adductor longus region, and gluteal musculature. The narrow-AP/wide-ML shape forces the femur into non-anatomic abduction, reducing gluteus medius efficiency and producing the lateral trunk lean that defines quadrilateral gait.

The ischial containment (IRC) socket; also called CAT-CAM (Contoured Adducted Trochanteric-Controlled Alignment Method) or NSNA (Normal Shape Normal Alignment) — is the modern standard. The MAS (Marlo Anatomical Socket, Ortiz, late 1990s) is an evolution with low posterior and gluteal trim lines. The IRC cross-section inverts the quadrilateral: narrow ML, wider AP. The ischial tuberosity and ramus are contained INSIDE the socket; the medial wall captures the ischium and the lateral wall contours over the greater trochanter, creating a “bony lock” that resists rotation and provides ML stability. The femur is maintained in anatomic adduction, optimizing gluteus medius leverage. Socket flexion and adduction are each ~5-7 degrees. MAS adds a low posterior trim so the patient sits on gluteus maximus rather than the socket edge.

The critical board distinction: quadrilateral = ischium ON posterior brim, femur in abduction; IRC/MAS = ischium INSIDE socket, femur in anatomic adduction. IRC shows greater hip ROM (~140 vs ~127 degrees, p=0.01), lower energy cost, improved step length, less lateral trunk lean, better rotational control, and better seated comfort.

Flexible thermoplastic-within-rigid-frame sockets combine a thin thermoplastic inner socket with a rigid laminated outer frame. Advantages: sitting comfort, heat dissipation, proprioception, translucent inspection, volume-fluctuation accommodation, reduced weight.

Sub-ischial sockets place the proximal trim entirely below the ischial tuberosity, eliminating ischial containment for unrestricted hip motion. A systematic review of seven randomized or controlled trials (five comparing sub-ischial with ischial containment, with very low certainty for a comfort benefit and no established mobility advantage) and the HySS case series showed improvements in passive hip ROM, walking speed/distance, stride length, and reduced double-support time. Sub-ischial appears non-inferior to IRC and may offer better hip ROM and sitting comfort.