Principles of Orthopaedic Rehabilitation

Envelope of Function

Tissue response to loading has been described as the result of the magnitude and frequency of loading. When load and frequency are matched appropriately, the individual is functioning in his or her “zone of homeostasis,” which is loading that does not result in tissue adaptation or tissue failure ( Fig. 31.1 ). When the magnitude or frequency of loading exceeds the tissue's physiologic tolerance, the individual begins to function outside of his or her “envelope of function.” This results in supraphysiologic (i.e., stress that is greater than the normal stress of a given person) overload that is positive when the tissues can adapt. For example, when muscle experiences supraphysiologic load over time this results in hypertrophy and neural adaptation. However, when the load exceeds the maximum tissue tolerance threshold, either in magnitude or frequency, the result is structural failure. Following an injury or surgery, tissues are injured as depicted in the figure. This means that activities that were once within the envelope of function are now shifted outside of it. For example, prior to an anterior cruciate ligament (ACL) injury and reconstruction, walking 2 miles was likely within a patient's zone of homeostasis. However, in the weeks immediately following injury and surgery, this task would likely fall outside of the envelope of function. Thus at the beginning of a rehabilitation period, the tissues involved should be considered when establishing the loading and motion limitations to set the boundaries of the rehabilitation.

Acute: Chronic Workload Ratio

Workload represents all of the stress a given structure experiences ( Fig. 31.2 ). For example, femoral articular cartilage following a “bone bruise” would undergo stress not only from the rehabilitation exercises but also normal activities of daily living such as walking, going up/down stairs, and squatting to sit down/get up from a chair. Acute workload is most often defined as the workload over the 7 days, and chronic workload over the past 28 days. The relationship between the acute (what is currently imparted on tissues) versus the chronic workload (what stress the tissue has been able to adapt to) plays a critical role in the appropriate progression of individuals as they return to sport. Injury risk has been shown to be a function of the acute to chronic workload ratio, whereby higher ratios increase the likelihood of injury. Simply, acute dramatic increases in load compared with what the body is conditioned to respond to appears to increase injury risk.

The acute load is the load the individual has had over the past week, whereas the chronic load is the load the individual has had over the past 4 weeks. A load that had maintained a steady increase of 10% to 15% per week over time results in a “sweet spot” of the acute to chronic load ratio of approximately 1.3, which is an overall 30% greater load in the past 7 days compared with the past 28 days. In contrast, a recent spike in load, such as an increase of 30% in 1 week (increasing from 45 minutes running to 90 minutes, would result in an acute to chronic workload ratio of 2.0, which has been shown to increase an individual's risk of injury, which increases rapidly as the ratio increases.

This is important to consider in the rehabilitation process because workload should be increased gradually to allow tissue adaptation and decrease risk of injury. Throughout the rehabilitation process, this risk of injury can also be equated to risk of symptoms. If a patient has an acute spike in loading during rehabilitation, he or she may not have an injury but may return to the clinic with an increase in pain and swelling or a decrease in range of motion (ROM). These symptoms can be predictable based on which tissues are stressed.

Often athletes are injured in the first few months after return to sport, which may be due in part to too large a spike in acute workload relative to their chronic workload during the rehabilitation process. Optimal rehabilitation should progress load gradually from surgery through to full return to activity, with coordination between all providers.

Matched Dosing of Internal and External Load

With those models illustrated, it is important to consider how to apply load, how tissue responds, and how these changes lead to clinical decision-making. External load is what is typically thought of in rehabilitation, encompassed by the sets, reps, and resistance people use to perform an exercise or daily activities. Internal load is the tissue's response to an external load and can be viewed based on symptoms, including pain, swelling, change in ROM, etc. Matched dosing is a concept that is critical to successful rehabilitation, meaning that the external load should be matched based on the internal load with which a patient presents. External load should be increased systematically and gradually, based on bath knowledge of tissue healing, as well as tissue response to loading. If a patient arrives to a rehabilitation session having lost 5 degrees of knee extension ROM with an increase in pain and swelling after jogging last session, this may suggest the internal load is too high, with tissues giving clean warning signs of external overload. This patient's external load should be decreased until the symptoms of internal load decrease. Clinically, simple methods to measure internal load of the patient include pain, ROM, and rating of perceived exertion (RPE) ( Fig. 31.3 ). The Borg RPE scales are the most commonly used and have been shown to reflect heart rate, breathlessness, and muscle fatigue following exercise.

Tissue Healing Considerations

To prescribe the proper loading plan, understanding the basic tissue biomechanical properties is at the core of forming an effective plan of care in rehabilitation, whether nonoperative, preoperative, or postoperative. Tissues differ in both their ability to heal following injury and in their response to loading. A rehabilitation provider is equipped with the knowledge of these healing capacities and loading responses. This allows them to both unload tissue to the degree required to allow maximal healing while minimizing deleterious effects as well as load the tissue to the degree required for optimal return of function. An understanding of healing capacity and loading response dictates the degree to which specific tissues should be subject to forces across the continuum of loading based on both injury and surgery. Proper communication between a surgeon and rehabilitation provider regarding tissue pathology and surgical procedure allows the rehabilitation provider to incorporate this information into a patient's plan of care so they are loaded and progressed appropriately at an adaptive level through their course of rehabilitation.

Tissue healing is an important consideration in the development of time-based guidelines in a protocol. Some tissues affected during surgery require a period of immobilization for healing, whereas others can withstand immediate ROM. Likewise, some tissues benefit from limited or no weight bearing for a period of time, whereas others both withstand and benefit from immediate full weight bearing. There are additional patient-specific factors that may influence the timeline for postoperative restrictions, some of which are patient health, tissue quality, comorbidities, etc. A rehabilitation provider will expect instruction from a patient's surgeon regarding any limitations he or she may have postoperative for tissue healing, including the amount and time frame, regarding weight bearing, immobilization and ROM, and muscle activation. This information should be included in an established postoperative protocol, as well as the patient's postoperative referral to a rehabilitation provider if any modifications from the established protocol are made.

Key Tissue-Specific Considerations