Currently the OPRA^TM implant system (Figure 1) requires two surgeries. Stage 1 surgery involves the intramedullary implantation of a threaded and self-tapping titanium fixture into the residual femur accompanied by closure of the soft tissue and skin. Once skin and soft tissue have recovered and healed, at approximately 4 to 6 weeks after surgery, a traditional socket prosthesis may be used for increased mobility. Although a socketed prosthesis is allowed at this time, very few choose this option. If this option is pursued, distal contact in the socket should be kept at a minimum to protect the fixture and encourage healthy bone integration.^3,4 A distal gel pad or other soft materials may be used to help managing distal contact.

During stage 2 surgery the muscles are reattached to the periosteum, redundant soft tissues are removed, and a skin flap is attached directly to the distal aspect of the femur, creating an exit site for the abutment. The percutaneous abutment is then fit into the titanium fixture and is held in place by the abutment screw (Figure 2). The abutment serves as the attachment point to connect the prosthetic device.^5,6

Stage 2 surgeries originally occurred approximately 6 months after stage 1.³ However, efforts have since been made to safely decrease this time. Clinical trials at both Walter Reed National Military Medical Center and University of California, San Francisco, have reduced the time between the stage 1 and stage 2 surgeries to as little as 3 months.²

After stage 2 surgery, rehabilitation begins with the introduction of standardized weight progression protocols. These can be applied at normal or half speed depending on patient presentation. Normal speed is the standard timeline of progression, whereas half speed is for individuals with less than ideal skeletal conditions. During the normal speed protocol, once cleared by the surgeon, axial weight bearing is initiated on a scale no earlier than 4 weeks after stage 2 using a short training prosthesis (STP)² (Figure 3). Under the slower speed protocols the progression of rehabilitation is slower, and the weight applied by the patient is decreased.³

Standard prosthetic componentry can be used to assemble the STP using a four-hole connector with Integrum’s custom attachment device. The attachment device secures to the abutment using an Allen key. This STP does not have any features built into it to protect the interosseous fixture against excessive forces like that of a fail-safe device.⁵ Therefore, any twisting/torque should be avoided and weight bearing limited to strictly axial loads. The overall length of the STP should approximate the anatomic knee joint center of the contralateral side to allow the patient to kneel to initiate controlled weight bearing.

In the rehabilitation protocols described by Hagberg and Brånemark³ after fitting with the STP, weight bearing is initiated with 20 kg (approximately 40 lb) of loading, as tolerated, for 30 minutes twice per day. The patient can use a standard home bathroom scale to monitor their progression (Figure 3). Weight is increased as tolerated, but no more than 10 kg (20 lb) per week. Pain may be reported during this early stage of rehabilitation and often occurs when offloading the prosthesis. Pain reported at 2 to 3 on a standard 10-point visual analog pain scale is considered safe. Pain ratings above 5 points should be avoided, with the patient returning to lower axial weight loads. This allows the interosseous structures to recover before increasing to the higher loads.
The patient should keep a daily record of pain or discomfort level for each loading session.^3,5

Once the patient has progressed to full weight bearing within acceptable levels of discomfort using the STP, they can progress to a full-length articulating prosthesis. The connection between the full-length prosthesis and the abutment is achieved by using Integrum’s OPRA AXOR II device, which attaches to standard prosthetic componentry using a standard four-hole adaptor (Figure 4). The AXOR II device is donned and doffed by rotating the outer ring (Figure 5, A), which opens up the adjustable jaws of the device allowing for the abutment to seat directly into the AXOR II (Figure 5, B). The outer ring, assisted by an internal spring, engages the jaws onto the abutment and then is hand tightened to secure in place^3,5 (Figure 5, C).

The AXOR II also serves as a failsafe device. Excessive axial, transverse, or torsional loads or a combination of these loads could cause the bending and/or twisting of either the implant, the abutment, or both during an unexpected fall. The function of the fail-safe device is to protect the intramedullary implant, the abutment, and the retaining screw against these excessive loads. It is designed to release in flexion or rotation either individually or contemporaneously. Flexion release occurs when the prosthetic knee reaches its maximum flexion angle or end point, causing the device to open up (Figure 6). Rotational release can occur when an excessive torsional load or torque is placed on the device. This setting comes predetermined by the manufacturer. Although a rotational failsafe mechanism has been engineered into other implant systems, the flexion failsafe mechanism is unique to the AXOR II.

The design of the AXOR II allows the patient to reset the fail-safe mechanism without using any tools or having to replace any parts. Resetting is recommended while sitting down and can be achieved with the prosthesis still attached. If released in rotation the knee can be manually rotated back into place. When resetting the flexion, there can be an abrupt snap back into place. This may cause some discomfort early in the rehabilitation phase. Therefore, taking the prosthesis off the abutment before resetting it manually may be recommended. Once the AXOR II has been reset, the patient can reengage in ambulation. If there is increased pain associated with any fall or excessive load, the medical team should be notified as soon as possible.