The currently accepted standard for rehabilitation and mobility following amputation is a socket-mounted prosthesis. Osseointegration is an alternative method that has gradually gained greater acceptance in the last 30 years. It is defined as a procedure in which a metal implant is directly anchored to the residual bone, attached to a prosthetic limb using a transcutaneous connector.

The advantages of osseointegrated prostheses over conventional socket prostheses include stable fixation, significant increases in walking ability, range of motion and control of the prosthesis, and health-related quality of life. Moreover, bodyweight distribution results more similar to physiological conditions.

No formal consensus exists for osseointegration surgery. However, based on the positive clinical experience, surgeons currently indicate this surgery for those patients who show poor tolerance of socket prostheses.

The present study investigates neuro-physiologic and mechanical parameters of walking and balance in patients with lower limb amputation and osseointegrated prostheses and in matched patients with traditional socket prostheses to highlight strengths and weaknesses of the alternative technique with respect to the present standard of care.

The primary endpoint is the investigation of the neurologic and mechanic adaptation in terms of a) kinematic and dynamic segmental analysis of walking and transfer of the body center of mass during walking; b) capacity to retain balance in response to different conditions of oscillation, tilt, and translation of a posturographic platform.

The secondary endpoint is investigating of adaptation to walking on a split-belt treadmill mounted on force sensors with the belts running at different velocities.

We hypothesize that:

• the deficit in joint power of the prosthetic limb is associated with a phenomenon of "learned non-use" both in balance and during gait. This behavior looks automatic and unconscious. It consists of the under recruitment of the impaired side as a form of unconscious protection, which is adopted when the contralateral side may be exploited to carry out the function;
• the joint power provided by the prosthetic limb may increase both by increasing treadmill velocity and by walking in split-belt modality with the prosthetic limb on the faster belt;
• an "after-effect" will be evidenced after the split-belt walking test when the two belts will return to the same velocity; patients with osseointegrated prostheses and patients with socket prostheses may show different behaviors in the adaptation to split-belt walking and the following post-adaptation, as a result of the residual proprioception of the amputated limb.

Results from the present study will allow:

• the identification of the possible advantages in walking and balance symmetry in patients with osseointegrated prostheses with respect to patients with socket prostheses;
• the estimate of the sample size for future experimental protocols and new rehabilitative programs.

Inclusion Criteria:

• unilateral lower extremity amputation;
• age > 18 years;
• amputation-adjusted Body Mass Index between 18 and 25;
• ability to understand the instructions;
• ability to wittingly sign the informed consent form.

Exclusion Criteria:

• other previous knee injuries or major surgical interventions at the lower limbs;
• comorbidities, such as neurological conditions, vascular diseases, or diseases of orthopedic, cardiac, or pulmonary origin;
• cancer disease in the site under examination;
• other neurological pathologies with impact on balance and gait.