Introduction

The primary goal of anterior cruciate ligament reconstruction (ACLR) is to restore stability without sacrificing mobility or strength. The primary purpose of ACLR rehabilitation is to restore mobility and strength without sacrificing stability. It is the central hypothesis of this chapter that overly aggressive rehabilitation is both unnecessary and potentially sacrifices knee stability. The stability-conservative rehabilitation protocol can best be summarized as follows: Avoid graft strain and abrasion while restoring motion and strength in the early postoperative period.

It is based on the following premises and principles:

History

Until the late 1980s, ACL rehabilitation was rendered cautiously on the theory that stability was fragile and strain in the graft needed to be minimized in the early postoperative period to avoid loss of stability. It was well known that grafts lose much of their tensile strength in the first few postoperative months.^1–3 It was recognized that grafts require time to heal into bone tunnels before which they are subject to loosening^2,4 (see Chapter 56). Finally, it was also known that quadriceps contractions in the terminal 50 degrees of extension exerted a powerful anterior translational moment on the tibia that strains the ACL.^5,6 As bone–patellar tendon–bone (BPTB) grafts became popular during the 1980s, it became apparent that many knees became stiff and had prolonged quadriceps weakness. This stiffness was potentially a worse problem than the laxity of ACL deficiency. A stable but stiff knee was more likely to be worse than an unstable knee. In 1990 Shelbourne published his classic paper⁷ introducing accelerated rehabilitation. This challenged the then-accepted view that grafts needed to be carefully protected during the first few postoperative months. He stated that his patients who were somewhat noncompliant with his postoperative restrictions and were more active had no greater incidence of instability than the compliant patients. However, these more active patients had less stiffness and weakness. He postulated that “accelerated” early aggressive rehabilitation was thus not harmful and also necessary to ensure restoration of motion and strength. Based on these observations he developed his accelerated rehabilitation protocol, which is currently used in some form by most ACL surgeons.

Symmetric Stability After Anterior Cruciate Ligament Reconstruction is Not Assured

Many surgeons commonly use exercises such as cycling and squats that cause quadriceps contractions in the terminal 50 degrees of extension in the early postoperative period. As shown by the research of Beynnon and others^8,9 (see Chapter 64), these activities exert a significant strain on the ACL. The accelerated rehabilitation protocol states that stability will not be compromised by such ACL-straining activities if the surgery is properly performed. Yet, as shown in the most recent meta-analysis¹⁰ to review all such papers and as discussed elsewhere in this book (see Chapter 69), symmetrical stability after ACLR is currently achieved in only about half of all reconstructed knees, even in the hands of the very experienced ACL surgeons. Accelerated rehabilitation was developed, in particular, to overcome stiffness and weakness in BPTB patients. However, mean KT-1000 scores for BPTB from the literature showed that 34% of reconstructed knees had more than 2 mm of increased laxity than the normal knee, the stability level usually seen with a partially torn ACL. Of the 66% that were within 2 mm, it is estimated that one-fourth or 17% had exactly 2 mm of increased laxity. Thus 34% + 17% or 51% in all had 2 mm or more increased laxity. This leaves only about half with restoration of stability that is the same as the opposite knee. In addition, 5.9%, or roughly 1 in every 17 knees, had abnormal stability (i.e., a failed graft). Thus some of the most experienced knee surgeons in the world in the current literature are only restoring even approximately symmetrical stability—within 1 mm of the other knee—to half of the operated BPTB knees. Half have stability at a level seen with a partially torn ACL or worse.

Why Protect The Graft in The First 3 Months Postoperatively?

Muscular Inhibition After Anterior Cruciate Ligament Reconstruction

Effusions,^11–13 ACL injury,^14,15 and knee pain^16,17 have all been shown to have an inhibitory effect on knee musculature, especially the quadriceps, apparently through afferent inhibition of motor neuron activity. This means that during roughly the first 3 postoperative weeks, when most patients have a large hematoma and some knee pain, it may be difficult or impossible to generate large-enough muscle contractions to strengthen or effectively prevent atrophy. In the past when we attempted to begin strengthening immediately after surgery, we noticed that strength gains did not occur, probably for this reason. In addition, the exercises produced significant patient discomfort. For this reason we no longer begin strengthening until roughly the end of the third postoperative week for isometric quadriceps and later for other muscle groups in which atrophy is less of a concern.

Cyclical Loading Does Cause Laxity

Every study that has looked at this subject has shown that cyclical loading induces elongation of the graft–fixation construct. These studies typically show several millimeters of elongation.^18–20 The best such study showed 1 mm after the first 100 cycles. Many of these studies look only at the first 1000 cycles.¹⁸ One thousand cycles is equal to the number of steps taken in less than 1 week of normal activity and less than 1 week of cycling during rehabilitation. It is highly likely that this elongation would further increase if more cycles were performed. It is important to realize that only 2 mm of elongation from cyclical loading is enough to change the side-to-side difference from –1 mm to –3 mm, converting good stability to the stability level seen in a partial ACL tear.

Why Avoid Hyperextension?

The studies of Beynnon (see Chapter 64) have shown that hyperextension strains the graft significantly. Thus hyperextension should be avoided for that reason. Also, hyperextension is not used during normal gait. In our large long-term study²¹