Prosthetics in Orthopedics

Function

Lower Limb Prosthetics 

Lower extremity amputations are not uncommon in the United States.  Approximately 110,000 people are subject to some level of major (excluding toes) lower limb amputation each year.[2] Up to 70% of lower limb amputations result from a disease state, most of which are due to vascular disease and diabetes. The remaining lower limb amputations are the result of trauma, congenital abnormality, and tumor. Furthermore, the majority of these amputations are transtibial or at a distal site; transfemoral are less common. Interestingly enough, of the 85% of amputees fitted for a prosthesis, only 5% use the prosthetic limb for more than half of their daily walking.[3]

The Center for Medicare and Medicaid Services (CMS) has created a classification system to help guide practitioners and prosthetists to select the appropriate componentry based on their potential to be successful with the prosthesis. This is called the K-Classification System for Functional Ambulation and is often referred to as a patient’s “K-level.”[4] This is especially important to consider in patients with CMS payer status not to place an undue burden on families if the particular prosthesis recommended is not covered by insurance.

The K-Classification for Functional Ambulation[4]

• Level 0: Does not have the ability or potential to ambulate or transfer safely with or without assistance, and a prosthesis does not enhance their quality of life or mobility.
• Level 1: Has the ability or potential to use a prosthesis for transfers or ambulation on level surfaces at fixed cadence—typical of the limited and unlimited household ambulator.
• Level 2: Has the ability or potential for ambulation with the ability to traverse low-level environmental barriers such as curbs, stairs, or uneven surfaces—typical of the limited community ambulator.
• Level 3: Has the ability or potential for ambulation with variable cadence.  Typical of the community ambulator who has the ability to traverse most environmental barriers and may have vocational, therapeutic, or exercise activity that demands prosthetic utilization beyond simple locomotion.
• Level 4: Has the ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact, stress, or energy levels—typical of the prosthetic demands of the child, active adult, or athlete.

The goals of a lower limb prosthesis include it be comfortable, lightweight, durable, aesthetically pleasing, low maintenance, and provide an appropriate degree of mechanical function for the amputee’s K-level. The major components of a lower extremity prosthesis include the socket, suspension mechanism, knee joint (if needed), pylon, and the terminal device.

The socket is the portion of the prosthesis that encompasses the residual limb.  The initial socket may be a temporary one, sometimes referred to as a preparatory socket. It can be formed with the use of a plaster molding of the residual limb as a template. It is used in the acute setting after amputation as swelling continues to decrease and the surgical incision heals.

The prosthetic socket serves several important roles. It protects the residual limb but also allows for weight-bearing and load distribution. The most commonly used socket today is a patellar tendon-bearing (PTB) prosthesis. This socket is used specifically for transtibial amputations. More modern designs have incorporated hydrostatic loading to more evenly distribute the load through the residual limb, also known as a total-surface bearing.[5] These designs help prevent skin breakdown and are more comfortable for the amputee.

The suspension mechanism attaches the prosthesis to the residual limb. This can be accomplished with the use of belts, wedges, locks, and/or suction. Some suspension designs are created using a hybrid of the aforementioned elements. The two types of standard suspension mechanisms are locking and suction, each of which utilizes a silicone-based sock applied over the residual limb, which is then inserted into the socket.  The locking system utilizes a pin or strap adhered to the silicone sock and a distal mechanism that fastens to the pin or strap respectively.  A suction suspension system utilizes a similar silicone sock with a one-way expulsion valve and sealing sleeve on the socket to create an air-tight seal, stabilizing the limb from the proximal seal downward.   

An articulating knee joint is sometimes appropriate for the prosthesis. These may consist of a single axis simple hinge joint or a polycentric axis with multiple centers of rotation.  The simplest and most commonly used mechanism is the single axis hinge joint, of which the primary function is to provide articulation, allow knee flexion in swing-phase, and resist knee flexion during weight-bearing.[6]  A polycentric knee joint incorporates four and six-bar mechanisms to enhance stance-phase stability and swing-phase kinematics. Although highly successful in developed countries, the cost and complexity associated with polycentric knee joints make them limited in developing countries.

Even more expensive modern designs have made use of microprocessor-controlled hydraulic knee joints that provide more reliable control when ambulating at different speeds, going up and downstairs, and walking on uneven surfaces.

The pylon or “shell” portion of a prosthetic is what attaches the socket to the terminal device. It can also be referred to as the containment socket. Recent advances in prosthetic technology have paved the way for dynamic pylons that permit axial rotation and absorb energy from the residual limb. These can be endoskeletal or exoskeletal, whichever is more functionally appropriate and aesthetically pleasing for the amputee.

The terminal device is the last piece of the prosthetic puzzle. This is typically a traditional looking foot, but more customized devices exist for high-level athletes. Ankle function is typically built into the terminal device, however, separate ankle joints may be beneficial in some patient populations. The drawback to these higher-functioning prostheses with ankle joints is the added weight to the distal end of the prosthesis.  This added weight requires more energy expenditure and limb strength to control the additional degree of freedom.

The prosthetic foot serves to provide a stable surface, absorb shock, replace lost muscle function, replicate the anatomic joint, and restore aesthetics.  Terminal devices can be broken down into non-energy-storing feet and energy-storing feet.

Non-energy-storing feet include a single-axis foot and solid ankle cushioned heel (SACH). The SACH has been extremely popular since its inception and uses a compressible material in the heel to mimic ankle plantarflexion, permitting a smooth gait.  It is a great option for the K-1 level ambulator or a sedentary patient with a transfemoral or transtibial amputation.[2] The single-axis foot adds passive plantarflexion and dorsiflexion, increasing stance phase stability.

Energy-storing feet include the multiaxis foot and the dynamic response foot.  The multiaxis foot adds inversion, eversion, and rotation to the traditional abilities of the single-axis foot.  The increased degrees of freedom of the multiaxis foot make it a great option for the K-2 and K-3 level ambulators who can perform even light to moderate level activity.

The dynamic response energy-storing foot is considered top-of-the-line when it comes to lower extremity terminal prosthetic devices and is reserved for younger, more athletic populations.  This device uses a flexible keel that deforms under pressure and returns to its original shape when the load is removed.[7] This allows for higher-level functions such as running and competitive sports participation.  These terminal devices are suitable for the K-3 and K-4 level ambulatory.

Upper Limb Prosthetics 

Upper extremity amputations are certainly rarer than lower extremity amputations. As of 2005, approximately 41,000 people in the United States were living with major (excluding hand and finger, etc.) upper extremity amputations.[1] This number is expected to triple by the year 2050, following the same trend as lower extremity amputations. Approximately 80% of major upper extremity amputations are the result of traumatic injury.[1] The remainder is secondary to vascular disease, tumor, and infection.

Transradial amputations are the most common amputation performed proximal to the wrist.[8] At this level, preservation of a least 5cm is important for prosthesis fitting.[9] Prosthetic fitting for upper extremity amputations can begin much quicker than for lower extremity amputations due to less concern for wound breakdown, with some centers fitting immediately postoperatively.[10] Early fitting in upper extremity prostheses can protect the stump site, help control pain and edema, and lead to improved outcomes.

Orthopedic surgeons and prosthetists should consider amputation level, expected functional outcomes, financial resources, aesthetic importance, and job requirements of the amputee when fitting for a prosthesis.  Like lower extremity prostheses, there are a variety of upper extremity prostheses available to patients. These include cosmetic, body-powered, myoelectric, and hybrid prostheses.[11]

Cosmetic prostheses are generally the most lightweight prostheses available and require the least amount of harnessing.[9] That being said, they provide the least amount of function for the amputee. Body-powered prostheses come at a moderate cost and weight but are the most durable on the market. They provide the most sensory feedback but are less aesthetically pleasing and require more gross limb movement. Myoelectric prostheses function by transmitting electrical activity from muscle contraction to surface electrodes on the residual limb. These electrical signals are then sent to the motor to initiate the function of the terminal device. These devices tend to be the most expensive prostheses available. They are heavier, provide less sensory feedback, and require the most training for amputees.  However, they do provide more functional use and are more aesthetically pleasing. Hybrid prostheses use a combination of myoelectrical devices and cables to perform a multitude of functions. Transhumeral amputees generally use these devices.

The goals of upper extremity prostheses are similar to lower extremity prostheses. The major components of upper extremity prostheses include the terminal devices, wrist units, elbow units, and shoulder units.

Terminal devices can be passive or active.  Passive devices are cheaper more aesthetically pleasing than active devices but provide less function.  Newer materials can produce prostheses that are nearly indistinguishable from a native hand.  Active terminal devices are more expensive but allow for more function.  They are generally divided into hooks and prosthetic hands with myoelectric devices and cables. There are 5 types of grips available that can be selected based upon desired prosthetic function. These include precision grips (pincer function), tripod grips (3-jaw pinch), lateral pinch grips (key pinch), hook power grips (carrying a briefcase), and spherical grips (turning a doorknob).  Hand-like devices are a good choice for patients that work in an office setting, while non-hand prehension, or grasping) devices are better for laborers—another factor to consider is whether a voluntary opening or a voluntary closing mechanism would best suit the patient.

Wrist components often come in several flavors. A quick-disconnect unit allows for a simple exchange of different types of terminal devices. This allows patients to perform a multitude of functions. A locking wrist unit is one that prevents rotation during lifting and grasping.  Wrist flexion units allow for flexion and extension.

Elbow units are generally either rigid or flexible. A rigid elbow hinge unit can be used when an amputee can still perform elbow flexion but lacks pronosupination. These devices can provide increased stability, especially in patients with a short transradial amputation stump. A flexible elbow hinge can be selected for patients who retain sufficient pronosupination in addition to flexion and extension. These devices are desirable for patients with a wrist disarticulation or long transradial amputation stump and provide more function for the patient.

For patients with amputations at the level of the shoulder, prosthesis fitting becomes much more difficult. The increased weight and energy expenditure of prostheses at this level lead many patients to choose a prosthetic that is purely aesthetic in nature. Cosmetic prostheses in this patient population improve self-image, confidence, and the fit of clothing.

Issues of Concern

Complications in Prosthetic Care 

Prosthesis usage is not without associated complications. One of the most notable complications is that of choke syndrome.  This syndrome is characterized by venous outflow obstruction due to a socket that is too snug and is more common in lower extremity prostheses. In the acute phase, the skin over the stump site becomes red and indurated and may result in an "orange-peel" appearance. The chronic phase may result if the constriction is not resolved and may result in hemosiderin deposits and venous stasis ulcers. 

Dermatologic problems are common in this patient population. Contact dermatitis may result and Is most commonly caused by liners, socks, and suspension mechanisms. Treatment involves removal of the offending agent and symptomatic treatment with topical diphenhydramine or steroid creams. Painful scarring may result as a complication of the surgical amputation but may be exacerbated by prosthetic wear. It is important to ensure an appropriate prosthetic fit. Additionally, amputees can massage and lubricate the scar to alleviate pain and improve scar healing. Finally, an inappropriate fit may also lead to excessive sweating and cyst formation. Again, it is important to ensure a properly fitted prosthetic to avoid these complications.  

Amputees may also experience what has been termed as a painful residual limb. This pain can be generated from heterotopic ossification at the bony amputation site, excess bony prominences, a prosthesis with a poor fit, neuroma formation at the site of nerve transection, and insufficient soft tissue coverage of the stump site. Bony-related pain generators are caused by excessive pressure at that site and can be treated with surgical excision if necessary. Poor-fitting prostheses can cause friction between the skin and the prosthetic socket and need to be refabricated. It is also important to consider the unstable residual limb in patients who did not have a myodesis performed at the time of amputation. Also, this would be much more difficult to treat. 

Energy Requirements of Gait 

It is important to consider the energy requirements of amputees who wish to continue ambulating with a prosthesis. In some cases, the increased metabolic demand can so much that it limits a patient's ability to ambulate after a lower extremity amputation. Transtibial amputations increase energy consumption relative to a normal non-amputee by an average of 25%. Bilateral transtibial amputations increase energy expenditure by 20% to 40%. Unilateral transfemoral amputations increase expenditure by 60% to 70%, while bilateral transfemoral amputations increase expenditure by over 200%. It is important to note that the average increase in metabolic demand is much higher in amputations secondary to vascular etioligies compared to those of traumatic injuries.

Clinical Significance

The ultimate goal of a prosthetic limb is to enable a patient to fully return to their routine activity. For younger patients, the goals may be activities of daily living and various sports.[12] These goals may be to return to work for older patients and have successful functioning at a job. A clinical evaluation of the significance of the prosthetic limb can be obtained from a questionnaire with a mixture of performance tests and self-reported quality questions.[12] 

Another clinically significant indicator is dynamic balance, as evaluated by a four-square step test, socket comfort, pain, and distance walked in a 6-minute test. Mobility assessments can be performed pre-fitting and post-fitting to check the speed of ambulation. A well-fitted prosthetic limb can have a huge positive impact on improving quality of life and patient care. Quality of life has been shown to significantly improve after lower limb amputation with prosthetic limb use while in the inpatient rehabilitation setting and after 3 months time.[13] Also, quality of life is highly associated with psychosocial factors such as body image and lower-limb pain.[13] In summary, prosthetic limbs can greatly impact patient care with improvements in function and quality of life.

Other Issues

Osseointegration 

Osseointegration is the direct attachment of a prosthesis to the underlying skeletal structure. It is a surgical technique that directly attaches the prosthetic to the underlying bone without the formation of fibrous tissue at the bone-implant interface.[14] An osseointegrated device provides a continuous structural connection to the external prosthesis. This technique bypasses any issues associated with a traditional socket design, eliminating complications of poorly fitting prostheses and dermatologic problems.[15] In addition to the decreased complications associated with osseointegrated prosthetics, many patients benefit from reduced energy expenditure, improved range of motion, walking ability, and sitting comfort.[16][17]

Enhancing Healthcare Team Outcomes

Prosthetic training for amputees is a significant part of their recovery and healing process. This is a multidisciplinary approach involving surgeons, clinicians, physical therapists, occupational therapists, and prosthetists. Training programs are center-specific but help teach patients how to don and doff their prosthesis, inspect their amputation site for wounds and skin breakdown, and perform safe transfers.  Comprehensive programs, including mental training combined with gait training, have shown improved outcomes in transtibial amputees.[18] [Level 2]

More advanced training sessions assist amputees with non-assistive weight-bearing and with more advanced weight-bearing on uneven surfaces and stairs.

Nursing, Allied Health, and Interprofessional Team Interventions

Interprofessional team interventions are essential.  Physical therapists, orthotists, and prosthetists are significantly involved in all aspects of preparing a patient's stump for prosthetic intervention, creating the prosthesis, and making sure the patient can use it properly.

Nursing, Allied Health, and Interprofessional Team Monitoring

Physical therapists, orthotists, and prosthetists must all monitor and communicate any issues related to a prosthetic limb. Interprofessional communication is vital to make sure each team member is informed when issues arise, such as a poor-fitting prosthesis or functional limitations of the patient, and the issues are addressed via a multidisciplinary approach.