Functional outcomes after anterior cruciate ligament reconstruction: unravelling the role of time between injury and surgery, time since reconstruction, age, gender, pain, graft type, and concomitant injuries
Authors: Niederer Daniel , Behringer Michael , Stein Thomas 
Published: March 2022

Background
A multitude of individual, temporal, injury- and surgery-specific factors impact an individual’s functional capacity during the rehabilitation, return to sports (RTS), and re-injury prevention processes after anterior cruciate ligament (ACL) reconstruction. These factors’ interaction and nesting are not fully understood yet.
Purpose
To provide a more holistic view on the course of functional abilities after ACL reconstruction.
Methods
Anonymized data were retrieved from a nationwide registry. In this cohort study, patients with an acute unilateral ACL rupture, with or without concomitant ipsilateral knee injuries, and having passed an arthroscopically assisted anatomic reconstruction were included. Potential factors were age [years], gender/sex, time since reconstruction [days], time between injury and reconstruction [days], concomitant intra-articular injuries (isolated ACL tear, meniscal tear, lateral ligament, unhappy triad), graft type (hamstrings, patellar, or quadriceps tendon autograft), and pain during each measurement (visual analogue scale 0–10 cm). Repeated assessments of a comprehensive battery of classic functional RTS test were performed in the course of the rehabilitation and return to sports: Range of motion (Flexion [degrees], “Extension" [degrees]), Joint position sense / kinesthesia (Angle reproduction error [degrees]), Dynamic Balance Composite score [cm] of the Y-Balance test), drop jumps (Knee displacement [cm]), Vertical hop (Hopping height [mm]), Speedy jumps (Duration [seconds]), Side hops (Number of hops [n]), single leg hop for distance (hopping distance [cm]). Repeated measures mixed models investigated the impact and nesting interaction of the potential predictors on the functional outcomes.
Results
Data from 1,441 persons (mean age 29.4, SD 11.8 years; 592 female, 849 male) were included. Several variables like time between injury and reconstruction, time since reconstruction (estimates for ndays ranged from + .05 for single leg hop for distance to + .13 for range of motion; p < .001), age, gender, pain, graft type (patellar tendon graft: estimates between + .21 for Y-balance and + .48 for vertical hop performance; p < .001) ,and concomitant injuries contribute to the individual courses of functional abilities after ACL reconstruction. The unimpaired leg was mostly influenced by sex, age, the time between injury and reconstruction (estimates between − 0.0033 (side hops) and + .10 (vertical hopping height), p < .001)), and time since reconstruction.
Conclusions
Time since reconstruction, time between injury and reconstruction, age, gender, pain, graft type, and concomitant injuries are not independent but nested interrelating predictors of functional outcomes after anterior cruciate ligament reconstruction. Knowing them and knowing their contribution value is helpful for the management of the reconstruction (early might be better) deficit-oriented function-based rehabilitation (time- and function based) and individualized return to sports strategies.