Historically, knee disarticulation was advocated in the preanesthetic era because of the efficiency of the procedure, the limited associated bleeding, and because the technique does not violate the medullary canal.^1,2 Various skin flaps, including equal AP, long anterior or posterior, and sagittal flaps have been described and used;³ however, each of these flaps consisted only of skin and subcutaneous tissue and lacked adequate padding to permit routine end bearing on the terminal residual limb, limiting the ability of the patient to successfully use a prosthesis.

The retention of the full femoral length represents both the principal disadvantage and advantage of amputation at this level. The distal residual limb can be bulky and some degree of knee-level discrepancy is generally inevitable, but a stable, potentially end-bearing terminal residual limb is created with a long lever arm and an intact native insertion for the adductor magnus. Numerous revisions and modifications to the procedure have been proposed to address its limitations while preserving at least some of the advantages. Stokes⁴ advocated supracondylar amputation with retention of the adductor insertion and fixation of the patella to the distal femur. Mazet and Hennessy⁵ advocated trimming the medial, lateral, and posterior condyles and removing the patella to decrease bulkiness; Burgess⁶ advocated removing the distal 1.5 cm of the condyles and the patella. Bowker⁷ combined the latter two techniques, effectively achieving a long residual femur following transfemoral amputation. Although each technique has its merits, none has gained widespread use, potentially because each compromises the end-bearing and socket suspension advantages of knee disarticulation without fully addressing the shortcomings.

One prosthetic advancement and one surgical advancement in the late 20th century resulted in a renewed interest in knee disarticulation as a common, or even preferred, amputation level for many patients. Lyquist⁸ introduced the Orthopaedic Hospital of Copenhagen prosthesis, which featured a four-bar linkage polycentric knee joint that provided excellent stability while reducing cosmetic mismatch and knee height discrepancy because the shank folded under the socket when seated. Modernized versions on this knee design are currently popular. Wagner⁹ described the use of gastrocnemius muscle belly with sagittal skin flaps as terminal padding to improve end-bearing comfort. Klaes and Eigler,¹⁰ as translated by Bowker et al,¹¹ first reported the use of a posterior myofasciocutaneous flap incorporating the gastrocnemius muscles and leaving the perforating circulation to the skin flap intact. Thus, knee disarticulation has been rediscovered by modern, outcomes-oriented amputation surgeons for its capability to restore stable, functional ambulation for many patients after trauma or tumor resection.

By retaining the weight-bearing femoral condyles and providing adequate distal padding, the modern knee disarticulation allows residual limb weight bearing
without discomfort. The somewhat bulky nature of the terminal residual limb facilitates enhanced socket fit and suspension, improving sitting comfort by allowing the proximal socket trim lines to be kept out of the groin and distal to the ischium. In addition, the intact adductor magnus insertion obviates concerns about myodesis performance, stability, and resulting adductor strength in those with amputation at the transfemoral level. The physiologic cost of walking with a knee disarticulation is midway between that of transfemoral and transtibial amputation levels.^12,13,14 Good results have been reported in many ambulatory patients after knee disarticulation, with potentially decreased phantom sensation and residual limb pain, as well as lower risk of revision surgery for complications such as heterotopic ossification after high-energy trauma.^15,16,17 Although worse Sickness Impact Profile disability scores were reported in the Lower Extremity Assessment Project (LEAP) study for knee disarticulations versus transfemoral amputations,¹⁸ 12 of the 17 patients with knee disarticulation in that study did not have viable gastrocnemius muscle, and the authors of this chapter do not advocate knee disarticulation in such patients. A more recent comparative study of 10 military unilateral knee disarticulation patients and 18 patients with transfemoral amputation found no differences in multiple patient-reported functional outcome measures.¹⁹

In addition, knee disarticulation improves the ability to transfer from bed to chair and optimizes seating balance for low-demand or nonambulatory patients with vascular pathology, diabetes, or other severe comorbidities. From a biomechanical perspective, direct load transfer, or end bearing, is more natural than in transfemoral or transtibial amputations, allowing potentially enhanced feedback and proprioception during upright activities^11,20,21 (Figure 1).

Prosthetic fitting of a patient with a knee disarticulation can take advantage of the four-bar linkage polycentric knee joint that provides intrinsic knee joint stability, decreasing the risk of stumbling and falling while minimizing knee-level discrepancies^22,23,24,25 (Figure 2). This enhanced intrinsic knee joint stability becomes important when the minimally ambulatory patient with unilateral transtibial amputation requires contralateral amputation. Such patients generally have limited prosthetic goals, making stability during walking even more crucial. When knee disarticulation is performed on the contralateral limb, these individuals demonstrate enhanced stability on that limb without an apparent loss of walking propulsion.²¹ Despite the obligatory kneelevel discrepancy with regard to shank length and sitting prominence, many higher functioning amputees perform quite well with newer, microprocessor knees.