Introduction

Proprioception, a sensory modality responsible for the sensation of joint movement and position, plays a crucial role in the afferent–efferent neuromuscular control arc and normal joint performance [26, 32, 41, 42].

Proprioceptors, including Ruffini endings, Pacinian corpuscles, and Golgi tendon organs, are located at the tibial bone insertion area of the anterior cruciate ligament (ACL) [1, 9].

Therefore, ACL injury can cause damage and loss of proprioceptive receptors (based on the time between injury and surgery [14]) and can translate into a decrease in afferent information input [15, 3).

Rehabilitation

Three studies [11, 19, 28] reported weight-bearing rehabilitation after ACLR. Partial weight-bearing and full weight-bearing exercises started at least 2 weeks and 6 weeks after reconstruction, respectively (Table 3). Rehabilitation protocols were identical for treatments and controls in all studies that provided such details.

Outcomes

Proprioception assessment

JPS test-RPP

Three of four studies [11, 19, 28] measured RPP at different follow-up times. Chen et al. [11] analyzed RPP test results at 3, 6, and 12 months after surgery and found that the ACLR-R group had significantly better RPP results than ACLR-S in all testing conditions (knee flexion of 15°, 30°, and 45°; P < 0.05). Two studies [19, 28] followed up on participants for more than 24 months (24–36 months). One of the two studies that used the sparing technique reported a statistically significant difference in RPP test, indicating better proprioception in ACLR-S (knee flexion of 15° and 30°; P = 0.40 and P = 0.010). The other study [19] analyzed RPP test results at 3, 6, 9, 12, 18, and 24 months, but presented statistically insignificant findings (P = 0.739) (Table 4). Interestingly, both studies reporting JPS-RPP improvement were observational studies, while the one study reporting no significant difference was a clinical trial.

Table 4 Results of proprioception per included study: joint position sense
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JPS test—RAP

Only one study [5] tested RAP before and after surgery with a mean follow-up duration of 7 months. The test results showed greater improvement of proprioception in ACLR-R compared with that in ACLR-S (P < 0.05) (Table 4).

JPS test—recording and testing

Three studies [11, 19, 28] recorded and compared the mean JPS value (test angle minus setting angle) of the reconstructed knees, while one study [5] recorded the inaccuracy of both legs (involved and contralateral normal knees) and reported side-to-side differences in the JPS value. Two studies used the Biodex system to measure JPS [5, 19], one study [28] used Thomas splint and Pearson attachment, while the remaining study [11] did not report on the testing apparatus. Only one study [19] described the test speed (with a speed of 5°/s) (Table 4).

TTDPM test

One study [28] measured TTDPM by continuous passive motion at final follow-up. Patients were tested at three angles of knee flexion with a speed of 0.5°/s. There was no statistically significant difference between ACLR-S and ACLR-R; however, the ACLR-R group showed better results at all angles (Table 5).

Table 5 Results of proprioception: threshold to detect passive motion
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Balance tests, knee stability, and patient-reported outcomes

Only one study [28] reported on balance or postural tests. They conducted the one-leg hop test and single-limb standing test and found a statistically significant difference between the two groups (P < 0.05). Regarding knee stability and patient-reported outcomes, only one study [5] reported significantly greater improvement in anterior laxity tested by Rolimeter after ACLR-R compared with ACLR-S (P < 0.0001). None of the remaining studies found a significant difference (Table 4).

Discussion

The most important observation of this review was that patients with ACLR-R showed improved postoperative proprioceptive evaluation results compared with those of the non-remnant ACLR-S. However, the long-term improvement of proprioception in ACLR-R remains unclear since the majority of studies failed to report long-term (> 16 months) follow-up results. Additionally, the heterogeneity of the characteristics and proprioceptive assessment of the studies prevented us from statistically evaluating the clinical outcomes.

Currently, there have been several meta-analyses or systematic reviews debating whether ACL tibial remnants should be saved during surgery [20, 25, 33, 34, 47, 48, 50]. Such reviews reported equivalent or superior postoperative clinical outcomes with ACLR-R compared with ACLR-S; however, there is insufficient scientific evidence supporting a definite conclusion. Moreover, these reviews [20, 25, 33, 34, 47, 48, 50] mainly concentrated on graft healing, synovial coverage, revascularization and ligamentization, knee stability function, and patient-reported outcomes, with a limited focus on proprioception or proprioceptive assessment. Therefore, our current review aimed to fill that gap by focusing on proprioception improvement.

Histological animal studies proved that ACL remnant preservation promoted new ingrowth of proprioceptors, neural cells, and nerve-related gene expression 6–12 weeks after surgery [23, 31, 45, 52], indicating the enhancement of proprioception of the knee joints in the early stage. The histological findings partially explained the results of our review, which reported a greater proprioceptive improvement in ACLR-R (compared with ACLR-S) in the short follow-up (≤ 12 months) period. Although there were a few findings of studies with longer follow-up that reported similar results, they lacked statistical significance [19, 28]. Histological studies in humans showed a reduction in the concentration of neural analogs in ACL grafts years after ACLR, regardless of graft source (allograft or autograft) [53]. Moreover, the effect of graft source on proprioceptive recovery has been unclear in several studies [7, 10, 39, 40] that have reported similar outcomes from ACLR with autograft, allograft, and artificial synthesis grafts. These results jointly indicate the potential benefits of remnant-sparing ACLR over the tensioning technique, and further comparisons of two techniques with different follow-up durations and graft sources in proprioception assessment and clinical outcomes are required in future studies.

Several human studies have evaluated the remnant-preserving effect after surgery with respect to remnant volume and surgical timing [29, 35, 44, 47]. However, the optimal volume and timing (time between the injury and the surgical procedure) for remnant-preserving ACLR in clinical practice require further investigation since only few studies reported the results of proprioceptive assessment. Only one study [19] (of those included in this review) described the mean time from injury to surgery. The varied descriptions of remnant volume in three included studies [5, 19, 28] also prevented us from performing subgroup analysis of the relationship between the remnant amount and proprioceptive restoration. Therefore, the effect of remnant volume and surgical timing during ACLR-R on proprioceptive recovery should be further studied.

Proprioception in this current review was mainly assessed with JPS (position sense) and TTDPM (movement sense). JPS is relatively easy to perform [37]. All studies included in the review reported on JPS [5, 11, 19], while only one study reported on TTDPM [28]. However, the two tests are commonly used for proprioception assessment, and both should be interpreted cautiously owing to the complexity of proprioception [37]. Furthermore, proprioceptors in the ACL and surrounding capsules and muscles [28] cannot be differentiated by any existing tests during assessment; thus, although JPS and TTDPM provided valuable information about joint position and movement sense, new tests are still needed for further investigation.

Limitations

This study has few limitations. First, only four studies (level of evidence II or III) were finally extracted and analyzed in the review, and heterogeneity in study characteristics and outcome measures was encountered. Thus, the results were qualitatively summarized. Therefore, high-quality studies with validated outcomes are required in the future. Second, studies that used ACL augmentation with selective ACL anteromedial or posterolateral bundle reconstruction were excluded from the review to reduce the risk of bias between ACL reconstruction and augmentation. Further studies with respect to the different remnant-preserving ACLR techniques are needed for further investigation. Third, publication bias might have existed because only online-published English-language articles were included.

Conclusion

The potential and benefits of remnant-preserving ACLR are apparent since improved results were observed in postoperative proprioceptive evaluation compared with the non-remnant standard ACLR.

More high-quality studies with validated tests are required to distinguish the effect of remnant preservation on knee proprioceptive restoration owing to the heterogeneity of existing studies.