Understanding Lower Limb Impairment Assessments Under the AMA 4 Guides

Dr David Slattery explains how to properly assess lower limb impairment using the AMA 4 Guides and why missing key steps could lead to inaccurate ratings.

When assessing lower limb impairment using the AMA 4 Guides, multiple different aspects of the Guides need to be considered by the specialist to correctly and comprehensively assess impairment. The lower limb components of the Guides are captured in chapter 3, “The Musculoskeletal System”.

The key consideration with lower limb impairment ratings is how to encapsulate all areas of someone’s impairment adequately and comprehensively. Critical to this is what aspects of impairment can be combined. This commonly raises questions.

There are various aspects of the Guides which can be utilised to assess lower limb impairment, including:

• Range of motion
• Gait Derangement
• Muscle Atrophy
• Weakness (Manual Muscle Testing)
• Joint Ankylosis
• Arthritis
• Limb Length Discrepancy
• Diagnosis-based Estimates
• Amputations

Each one of these components needs to be considered when a client is undergoing an examination; otherwise, they can be missed, and the client’s impairment rating may be incorrectly assessed.

The most commonly used aspects of the Guides for assessing lower limb injuries are range of motion and diagnosis-based estimates.

Range of motion

Chapter 3 of the Guides lists reference ranges for each joint’s range of motion. Specialists can measure the client’s range of motion during the examination, determine if there is a restriction of a joint range of motion relative to the published reference range, and then use the Guides to calculate an impairment rating from this.

Diagnosis-based estimates

Table 64 of the Guides lists various lower limb conditions, such as fractures, joint replacements, malunions, ligament injuries and deformities. Each of these has a prescribed impairment rating which varies depending upon the severity of the impairment. These range widely, from 1% whole person impairment (WPI) for a partial meniscectomy, up to 30% WPI for a poorly functional total hip or knee replacement.

Generally, diagnosis-based estimate impairments can be combined with other areas of impairment in the Guides, such as vascular disorders and neurological disorders.

In comparison, diagnosis-based estimate impairments generally cannot be combined with impairment ratings obtained from other evaluation methods of lower limb impairment. However, an exception to this can include if the impairment ratings were in relation to different muscle groups.

These rules apply to all evaluation methods of the lower extremities listed above.

Gait impairment

Another evaluation method is gait impairment, which is taken as a stand-alone measure of lower limb impairment.

Table 36 of the Guides defines gait impairment ratings. This ranges from 7% WPI for a patient with a limp with coexistent arthritis, up to 80% WPI for someone who is wheelchair dependent. Arthritis is a prerequisite for assessment of mild impairment using this table; however, for moderate and severe impairment categories there is no requisite diagnosis of arthritis.

Arthritis

Table 62 of the Guides defines arthritis impairment ratings. One aspect which is commonly used for lower limb impairment is crepitation, which is audible or palpable clicking during motion of a joint. This can sometimes provide additional impairment.

When assessing a client for a severe lower limb joint disorder it is common to observe post-traumatic arthritis. This is assessed using measures of cartilage thickness on x-ray. This can sometimes provide a significant impairment rating.

Interestingly, for the hip and knee, if a patient has 0 mm of cartilage thickness, Table 62 of the Guides provides a 20% WPI rating. This is greater than the impairment from a well-functioning hip or knee replacement, which rates at only 15% WPI under the diagnosis-based estimates aspect of the Guides.

Atrophy

Table 37 of the Guides defines atrophy impairment ratings. Up to 10% WPI can be assessed for a difference in calf or quadricep muscle bulk.

Atrophy is a very common consequence of lower limb conditions. This happens when the limb has been immobilised due to casting, bracing or persistent disuse.

Other conditions

Specialists need to consider spinal and pelvic conditions, in case they overlap with lower limb conditions.

Lower limb impairments from trauma should also be considered with skin and neurological injuries.

These may be combined to provide a higher overall combined WPI rating. Stay tuned for when we discuss this further in future blog posts.