Introduction

Forced hyperflexion, forced hyperextension, varus or valgus stresses, knee dislocations, high-intensity sports, or injuries from traffic can all contribute to sustaining a Posterior Cruciate Ligament (PCL) tear. Much is known about the best-practice rehabilitation for Anterior Cruciate Ligament (ACL) tears as they are often seen, while PCL tears are much less common and occur more frequently with other concomitant knee injuries. The presence of isolated PCL tears in particular is rare as Yoon et al., (2023) found that up to 95% of PCL tears are associated with other ligament injuries. The general advice given to people with PCL tears is to avoid posterior translation of the tibia early in the rehabilitation to allow healing of the tissue, followed by a progressive restoration of the knee range of motion and a strengthening program. Since promising results are obtained when adding balance training to the rehabilitation of ACL tears, the question arises whether this could also be beneficial in the rehabilitation of isolated PCL tears. This study gives us valuable insights into the rehabilitation of PCL tears by examining the addition of balance exercises to strength training for isolated Posterior Cruciate Ligament rehabilitation.

Methods

In this study, the effect of 12 weeks of balance training and strength training in isolated posterior cruciate ligament tears was studied. Those who sustained an isolated PCL tear (MRI-confirmed), had a positive posterior drawer test, were symptomatic when doing daily activities, and had no other leg injuries could be included. The injury had to be at least 3 months old.

The intervention they received consisted of a 12-week balance training program of two 1-hour sessions per week. These rehabilitation sessions consisted of a 15-minute warm-up on a stationary bike (70 rpm), 20 minutes of muscle strengthening (knee extension and knee curl exercise at 70% of 1RM for 2 sets of 12 repetitions), 15 minutes of BOSU balance exercises, and 10 minutes post-training stretch.

This program was divided into 3 phases: initial from weeks 1 to 4, intermediate phase from weeks 5-8, and late from weeks 9-12. This was mainly to adapt the balance exercises and to adapt the training intensity of the strength exercises. In the initial phase, the exercises were designed to train whole-body balance and double-limb stability.

In the intermediate phase, the exercises progressed to double-limb exercises with the eyes closed. The balance exercises from the late phase were performed in a single-leg stance.

To compare the effectiveness of the addition of balance exercises to strengthening, a control group was included. This group consisted of people who had received PCL reconstruction more than 2 years ago and who had no restrictions in their daily activities. They received, however, no rehabilitation training.

The outcome measures included the Lysholm score and the International Knee Documentation Committee score (IKDC). These are two patient-reported functional outcomes. For the first, scores are considered poor (<65), fair (65-83), good (84-94), or excellent (95-100).

Next to these patient-reported scores, an active and passive reproduction of a passive position test using a Biodex machine was performed to rate the proprioception in the affected and unaffected legs as well as to test quadriceps and hamstring strength. Knee laxity was examined with an arthrometer.

Results

A small sample of 10 subjects was analyzed at the 12-week follow-up in the balance group, while 9 subjects were analyzed in the control group. Table 1 indicates the equivalence of both groups at baseline.

Before the 12-week posterior cruciate ligament rehabilitation, the mean Lysholm score was significantly lower than the functional outcome score in those who had received prior PCL reconstruction (59.30 (± 19.49) versus 83.20 (± 13.18)). This difference disappeared after the 12-week follow-up since the score improved to 82.20 (± 11.94). The same was seen concerning the IKDC score which increased from 56.30 (± 18.07) to 79.20 (± 12.40), which equaled the scores of the functional outcomes in the post-PCL reconstruction group 79.90 (± 7.20).

The significant differences that existed between the groups at baseline did disappear after the 12-week balance and strength program was completed. The subjects completing the balance program indicated that they felt confident to return to pre-injury physical activities.

Questions and thoughts

It was not specified which of the outcomes was the primary outcome measure. Considering the patient-reported outcomes IKDC and Lysholm were discussed first in the paper, I suppose these were the outcomes of interest. Therefore, I discussed these in the Results section.

Considering the other outcomes, the participants achieved gains in isokinetic strength of the quadriceps and hamstring muscles throughout the 12 weeks of the strength training and balance program, except for the isokinetic strength at a velocity of 240°/s. Explosive strength may not have been addressed extensively. When the comparison was made for the noninjured leg before and after the 12-week training program, no improvement was observed. Was this the result of the strength training program being performed on the injured side only? Unfortunately, the authors did not specify whether the knee extension and knee curl exercises were performed bilaterally or unilaterally only. In case it was trained bilaterally, we would expect to see strength increases in the uninvolved leg as well. There remain two options, either the strength training was of too low load, or only the PCL-injured leg was trained.

The balance program combined with the strength training likely had a good effect on the participants’ knee proprioception. This was seen for the active reproduction of a passive position test. The passive reproduction of a passive position was not improved after the 12 weeks of training. Maybe this was because no specific reproduction exercises were included in the rehab, or was it because at baseline, there was no difference when compared to the control group? Either way, these results should be further tested.

No difference was found for knee laxity outcomes. It appears that the knee remained lax, yet as the participants indicated that they could return to pre-injury physical activities, it might mean that they improved confidence in their knee to an important level. It may also be that they had better control over the knee through the gains in muscle strength, even though this did not improve knee laxity. Still, several questions remain unanswered up to now.

Talk nerdy to me

The study did include a control group, but can we call this a control group? In any case, this study was not a randomized trial. Nor were the groups included at the same time. The comparability of the groups at the start of the study cannot be tested, but will probably show a different population (people who had surgery as long as 2 years ago versus recent PCL injuries). This makes it difficult and premature to conclude the value of adding balance training to regular strength training. To be correct, a study group performing the balance exercises on top of the strength training program should have been compared to a group performing the strength training alone. And ideally, then a true control group would have to be included.

So do we need to attach no importance to the results of this study? Given the little evidence that is available in isolated PCL injuries and the few studies that examine exercise therapy as the main intervention, I believe that these results may be a valuable start for designing better trials.

Take home messages

This was a study with a pre-post design that included participants who participated in a Posterior Cruciate Ligament rehabilitation program that consisted of balance exercises and strength training. The study showed that with 12 weeks of nonoperative exercise training similar outcomes can be achieved as participants who received PCL surgery 2 years earlier. Certainly, due to methodological limitations such as a small sample size, the lack of a true control group, and the absence of a randomized design, many questions remain. Nonetheless, this study shed light on a topic where not much is known about isolated Posterior Cruciate Ligament rehabilitation.

Reference

Lu CC, Yao HI, Fan TY, Lin YC, Lin HT, Chou PP. Twelve Weeks of a Staged Balance and Strength Training Program Improves Muscle Strength, Proprioception, and Clinical Function in Patients with Isolated Posterior Cruciate Ligament Injuries. Int J Environ Res Public Health. 2021 Dec 6;18(23):12849. doi: 10.3390/ijerph182312849. PMID: 34886588; PMCID: PMC8657930.