Purpose
To systematically assess published classification systems for knee ligament injuries and to propose a comprehensive, multiplanar, sequential knee ligament classification system that improves upon those in the published literature.
Methods
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. PubMed, Embase, and Cochrane were searched from inception to January 2025 for studies reporting knee classification systems. Inclusion criteria consisted of studies reporting knee classification systems and English language. Exclusion criteria consisted of classifications only describing specific type of knee injuries (e.g., only anterior cruciate ligament injuries). The characteristics of each classification system were recorded and analyzed descriptively. A classification system was proposed that addresses the weaknesses of the existing systems.
Results
A total of 6 classification systems were identified in 8 studies in the literature and were published between 1975 and 2009. Existing classification systems were strong in correlating type of injuries with rotatory ligamentous laxity and physical examination findings but were lacking assessment of knee position during injury, discussion of sequence of injuries, involvement of bone marrow edema (except one study), and lack of correlation with knee dislocations. No clinical validation was performed. A knee classification system was created that included sequence of injuries, correlation with knee dislocation and injury mechanism, and bone marrow edema.
Conclusions
Knee injury classification systems lack comprehensive consideration of injury mechanism, sequence of injury, bone marrow edema, meniscus and chondral damage, involved ligaments, rotatory instability, and correlation with knee dislocation. The sequence of injury to these structures could be relevant to understanding injury mechanism and treatment, but requires validation.
Level of Evidence
Level V, systematic review of level IV-V studies.
The knee joint has 6 degrees of freedom with 3 rotations and 3 translations. Four of these are clinically relevant with normal physiological movements (i.e., flexion-extension rotation, tibial external-internal rotation, varus-valgus rotation, and anterior-posterior-translation), of which 2 only occur minimally in the well-functioning knee (i.e., varus-valgus rotation and anterior-posterior-translation), and 2 do not occur because of anatomy and bony configurations (i.e., medial-lateral translation and axial translation). During traumatic injury to the knee, however, nonphysiological movements can be seen that cause ligamentous instability to the patients, such as external-internal rotation (e.g., increased pivot shift or dial test), anterior-posterior translation (e.g., positive Lachman or posterior drawer test), or varus-valgus rotation (e.g., positive valgus or varus stress test). ,
In isolated or combined injuries, instability patterns are generally classified and described by their affected ligaments (e.g., combined anterior cruciate ligament [ACL] and medial collateral ligament injury [MCL]) or by their combined instability pattern (e.g., anteromedial [AMRI] or posterolateral rotational instability [PLRI]). ,,,,, Attention has been paid at different times to uniplanar or biplanar rotatory instability, such as attention to isolated PLRI from 1990s to 2000s, anterolateral rotatory instability (ALRI) from 2013 to the present, and more recently AMRI. , These uniplanar or biplanar instabilities patterns are mostly viewed in isolation without attention to injury mechanisms, sequence of injury, incidence of different patterns, or the level of energy with injury mechanisms. ,, Understanding the sequence of injured structures, rather than just listing which ligaments are injured, might enable us to better understand how injuries occur and better treat patients. ,
In an expert consensus statement on mulitligamentous knee injuries published in 2024, it was stated that “an updated system based on the structures involved, and severity of their injury would be of value. There should not be an implied assumption of knee dislocation (KD). Instead, this should be stated separately in association with the nature of the injury mechanism (high, low or ultra-low velocity). Furthermore, the classification system should consider the extent of associated nonligamentous structures injured (including menisci, cartilage, and neurovascular structures). The purpose and categories of classification within this system should directly guide treatment decisions.” Therefore, the purposes of this study were to systematically assess published classification systems for knee ligament injuries and to propose a comprehensive, multiplanar, sequential knee ligament classification system that improves upon those in the published literature. It was hypothesized that many complete knee classifications would not include multiplanar instability incorporating sequence of injury, injury mechanism, level of injury, involved ligaments and bone marrow edema patterns.
Methods
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses were used for this scoping review.
Literature Search
On July 6, 2023, a systematic search was executed in the electronic libraries of PubMed, Embase, and Cochrane with the search algorithm (knee ligament injury OR knee ligament instability) AND (anteromedial OR anterolateral OR posteromedial OR posterolateral) AND (classification OR algorithm OR multiplanar OR combined) for all studies reporting on existing classification systems on multiplanar rotatory instability. All studies in the English language and since the inception were considered. All duplicates were removed. An update of the search was performed January 31, 2025, to ensure no new studies were missed.
Two authors (J.P.v.d.L. and R.A.G.H.) independently screened all studies by title and abstract first, and then reviewed full-texts of potential studies for final inclusion. For all studies, agreement was reached, and a third independent reviewer was not required.
Inclusion criteria consisted of studies reporting a classification system on all multiplanar rotatory instabilities, and English studies. Exclusion criteria consisted of studies only reporting on one type of rotatory instability (e.g., anterolateral instability, anteromedial instability, etc.), case reports and surgical techniques, or systematic reviews. Studies reporting a complete classification of rotatory instability presented in multiple studies were considered for inclusion. ,
Methodologic Quality of Studies
The methodologic quality of included studies was assessed using Methodological Index for Non-Randomized Studies criteria, of which only the first 8 criteria were used, because only noncomparative studies were included. The recommendation strength was assessed using the Grades of Recommendation, Assessment, Development, and Evaluation system, and the level of evidence was assessed using the adjusted Oxford Centre for Evidence-Based Medicine was used to assess the level of evidence.
Data Extraction
All classifications were assessed and collected in Excel 2021 (Microsoft Corp., Redmond, WA). Collected data included author names, year of publication, journal of publication, assessment of injury mechanism, involved ligaments, sequence of injuries, rotatory instability, correlation with physical examination, correlation with KD, and whether validation of the classification system was performed.
Nomenclature for Classification System
For the proposed classification system, structures of 8 stabilizers of the knee were used as these involve all possible knee ligaments and structures : anterior (ACL), posterior (posterior cruciate ligament [PCL]), medial (superficial MCL [sMCL]), lateral (lateral collateral ligament [LCL]), anteromedial (deep MCL [dMCL]), posteromedial (posteromedial capsule/posterior oblique ligament [POL]), posterolateral (posterolateral corner), and anterolateral (anterolateral capsule [ALC]) ). Rotatory instabilities were defined as AMRI, ALRI, posteromedial rotatory instability (PMRI), and PLRI. ,,, No validation or reliability analysis was performed for the classification system at this time.
Statistical Analysis
Because only a scoping systematic review of the available literature was performed, no statistical analysis was performed.
Results
Literature Search
A total of 1,138 studies were reviewed by title and abstract, of which 48 were reviewed by full text. A total of 6 classification systems were identified in 8 studies, as 2 classifications were presented in 2 parts ( Fig 1 ). ,,,,,,,
Preferred Reporting Items for Systematic Reviews and Meta-Analyses flowchart of included and excluded studies.
Methodologic Quality of Studies
Only 2 classification systems were clinical studies ,, and they were Level IV prospective or retrospective case series. Methodological Index for Non-Randomized Studies scores are displayed in Table 1 and were not performed for narrative review studies. The remaining 4 classification systems were expert opinion Level V evidence studies. ,,,,, There is a clear need for validation of classification systems using physical examination along with radiologic and biomechanical studies. The recommendations to use these classification systems is therefore “very low” using Grades of Recommendation, Assessment, Development, and Evaluation.
Table 1
Quality Assessment of the Included Studies Using the Methodological Index for Nonrandomized Studies (MINORS) Criteria
| Authors | Year | LoE | Study Design | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | Total |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hughston et al. , | 1976 | IV | Case series | 2 | 2 | 2 | 0 | 0 | 2 | 2 | 0 | 10 |
| Kennedy | 1978 | V | Narrative review | 1 | − | − | − | − | − | − | − | 1 |
| Hastings , | 1986, 1990 | V | Narrative review | 2 | − | − | − | − | − | − | − | 2 |
| Schenck | 1994 | V | Narrative review | 2 | − | − | − | − | − | − | − | 2 |
| Hayes et al. | 2000 | IV | Case series | 2 | 1 | 1 | 2 | 0 | 2 | 1 | 0 | 9 |
| Boisgard et al. | 2009 | V | Narrative review | 2 | − | − | − | − | − | − | − | 2 |
NOTE. The criteria of MINORS with 0 points when not reported, 1 when reported but not adequate, and 2 when reported and adequate. Maximum score is 24 for comparative studies.
1. A clearly stated aim: the question addressed should be precise and relevant in the light of available literature.
2. Inclusion of consecutive patients: all patients potentially fit for inclusion (satisfying the criteria for inclusion) have been included in the study during the study period (no exclusion or details about the reasons for exclusion).
3. Prospective collection of data: data were collected according to a protocol established before the beginning of the study.
4. End points appropriate to the aim of the study: unambiguous explanation of the criteria used to evaluate the main outcome which should be in accordance with the question addressed by the study. In addition, the end points should be assessed on an intention-to-treat basis.
5. Unbiased assessment of the study end point: blind evaluation of objective end points and double-blind evaluation of subjective end points. Otherwise, the reasons for not blinding should be stated.
6. Follow-up period appropriate to the aim of the study: the follow-up should be sufficiently long to allow the assessment of the main end point and possible adverse events.
7. Loss to follow-up less than 5%: all patients should be included in the follow-up. Otherwise, the proportion lost to follow-up should not exceed the proportion experiencing the major end point.
8. Prospective calculation of the study size: information of the size of detectable difference of interest with a calculation of 95% confidence interval, according to the expected incidence of the outcome event, and information about the level for statistical.
LoE, level of evidence.
General Analysis
Of the available classification systems, most of these systems discussed uniplanar and multiplanar (as discussed in the Introduction) rotatory instability and incorporated physical examination findings. Furthermore, most systems discussed which ligaments were involved. Only 3 systems included the injury mechanism in the classification, and only 1 study included bone marrow edema. Furthermore, no studies systematically incorporated KDs in their classification, and none of the studies addressed the sequence of ligament injury with injury mechanisms ( Table 2 ). No studies disaggregated their data by gender or race.
Table 2
Overview of Identified Studies in the Literature That Proposed a Multiplanar Knee Classification
| Author(s) | Year | Journal | Injury Mechanism | Knee Position At Injury | Sequence of Injuries | Bone Marrow Edema | Ligaments Involved | Multiplanar Rotatory Instability | Physical Examination | Central or Peripheral Origin | Correlation With Knee Dislocation |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Hughston et al. , | 1976 | JBJS | No | No | No | No | Yes | Yes | Yes | Yes | No |
| Kennedy | 1978 | Book | No | Yes | No | No | Yes | Yes | Yes | No | No |
| Hastings , | 1986, 1990 |
CORR
CFP |
Yes | Yes | No | No | Yes | No | Yes | No | Yes |
| Schenck | 1994 | ICL | Yes | Yes | No | No | Yes | No | No | No | Yes |
| Hayes et al. | 2000 | RG | Yes | Yes | No | Yes | Yes | Yes | No | Yes | No |
| Boisgard et al. | 2009 | OTSR | Yes | Yes | No | No | Yes | No | No | Yes | Yes |
CFP, Canadian Family Physician; CORR, Clinical Orthopaedics and Related Research ; ICL, Instructional Course Lectures ; JBJS, Journal of Bone and Joint Journal American Volume ; OTSR, Orthopaedics & Traumatology: Surgery & Research ; RG, Radiographics .
Part I: Existing Knee Classifications
In 1976 Hughston et al. described in 2 separate studies , the classification of medial- and lateral-based instability. For both studies, they extensively described physical examination tests to diagnose the types of instability, discussed their classification system, and then correlated their classification with surgical findings in 68 medial-based surgical cases and 89 lateral-based surgical cases, both in the acute and chronic setting. They divided instability into straight instability, rotatory instability, and combined rotatory instability. Straight instability was classified as medial, lateral, anterior, and posterior instability, and they describe the involved structures (sMCL/dMCL; LCL; and ACL and PCL, respectively) and corresponding physical examination tests. For rotatory instability, they described anteromedial, anterolateral, and posterolateral instability, which consists of combined injuries in the coronal and sagittal plane (ACL and MCL; ACL and anterolateral capsule; PCL and posterolateral corner, respectively). They emphasized their belief there is no isolated posteromedial instability, because there is no posteromedial instability with an intact PCL and no posteromedial rotatory instability with an injured PCL (but rather translational). Finally, combined rotatory instability was classified as combined anteromedial with anterolateral instability, posteromedial with posterolateral instability, anterolateral with posterolateral instability, or combined anteromedial with anterolateral and posterolateral instability, which can be explained by more extensive damage to mostly the medial or lateral structures causing the tibia to rotate in multiple planes and directions.
In 1980, Kennedy presented a classification system that was developed by several surgeons, including O’Donoghue, Moschi, Blazina, and Hertel as part of the Committee on Research and Education of the American Orthopaedic Society of Sports Medicine. They similarly divided instability into one-plane instabilities that include medial and lateral (i.e., coronal plane), and posterior and anterior (i.e., sagittal plane) instability, and they discuss which structures are damaged and which tests are relevant. They then present rotatory instabilities that include anterolateral, anteromedial, posterolateral, and also posteromedial instability, which they define as a combined instability in the coronal and sagittal plane that causes rotatory instability. Finally, they discuss combined rotatory instability that includes anteromedial with anterolateral instability, anterolateral with posterolateral instability, and combined anteromedial with posteromedial instability, which they define as more extensive injury of mostly the medial or lateral structures. In the discussion, they debate whether posteromedial rotatory instability exists in the acute setting and whether the PCL is involved or not.
In 1986, Hastings presented his classification of rotational knee injuries and focused more on anatomical structures rather than uniplanar or rotational laxities. He classified lesions as uniplanar, in which the 4 major ligaments (i.e., MCL, LCL, ACL, and PCL) cause instability, and combined 2-ligament instabilities, which are MCL-ACL, MCL-PCL, LCL-ACL, LCL-PCL, and ACL-PCL instability that causes biplanar (i.e. coronal and sagittal) instability. Finally, he discusses that 3 ligament combined injuries lead to uniplanar instability of severe valgus (ACL-PCL-MCL) and severe varus (ACL-PCL-LCL) rather than multiplanar combined instability, which is different from the other classifications. In 1990, Hastings further expanded his classification by adding components of injury mechanism, key symptoms, and signs with physical examination and corresponding (surgical) treatment. The classification is does not include order of injury and does not discuss rotatory combined instability.
Schenck presented his classification system specifically for KDs. He described which ligaments are injured after KDs and classified these as 1 cruciate ligament involvement (KD-1), both cruciates injured without collateral involvement (KD-2), both cruciates with at least 1 collateral involvement (MCL or LCL, which is KD-3M and KD-3L, respectively), and both cruciates with both collaterals injured (KD-4). Similar to the other classification systems, there is no validation, and it is rather a static observation of which ligaments are involved without correlation with physical examination, sequence of events, or clinical rotatory instability. This classification system is only focused on KDs and not knee injuries in general, but it has been most widely used and expanded to include more components of KDs. ,,,
In 2000, Hayes et al proposed a knee classification system on the basis of the mechanism of injury for noncontact knee injuries and on the basis of their information on ligament injuries, bone marrow edema patterns on magnetic resonance imaging (MRI), knee flexion position, force direction, and rotation presence. Furthermore, they assessed the incidence of these injuries on the basis of 100 MRI scans, but clinical information and instability were not included with this system. They noted that valgus stress in knee flexion with tibial external rotation was the most common injury type in 46% of cases, followed by hyperextension with varus movement (8%), knee flexion with tibial posterior force (8%), and pure valgus movement (6%). This classification system is the only classification system that considers the energy of trauma mechanism in which they indicate that more extensive ligaments are injured with more energy with the trauma mechanism.
In 2009, Boisgard et al. presented a knee classification that was co-authored by the French Society of Orthopedic Surgery and Traumatology. In their classification system, they divided injuries into 4 types and aimed to capture all bicruciate injuries and all dislocations regardless of how many cruciates are injured. Type 1 injuries are “simple” bicruciate lesions without dislocation, type 2 injuries are “pure” dislocations without collateral ligament injuries, type 3 are dislocations with single cruciate lesion, and type 4 are KDs with collateral injuries. Their classification is more focused on dislocations or bicruciate injuries and not on sequence of injuries, physical examination findings or correlation with concomitant injuries.
Part II: Proposed Classification
The current classification systems are lacking in validation, inclusion of MRI, and are not incorporating the sequence of events. The sequence of event might be important as this increases our index of suspicion (e.g., if a surgeon finds a Hill-Sachs lesion, one will look at the anterior labrum and bony inferior glenohumeral ligament lesions, or one will look for bony femoral neck prominence in the case of hip labrum tear) and will help surgeons to understand the etiology, which ligaments or structures are logically involved, and treat these ligaments. For example, there is current focus on involving dMCL reconstruction for ACL-MCL injuries due to a better understanding of the sequence of injuries. ,,,
Therefore, we have proposed a classification system on the basis of a combination of existing knee classifications, called the Classification of Ligament Injuries and Rotatory Knee Instability (CLIRKI) ( Table 3 and Fig 2 ). In CLIRKI, all facets of knee injuries are discussed, including the knee position during trauma, clinical (rotatory) instability, potential meniscus and cartilage impact (as evidence by bone marrow edema), and correlation with KDs ( Table 2 ). Also, this system approaches knee injuries as sequential ligament injuries that range from uniplanar injuries towards rotational instability and KDs.
Table 3
Proposed Multiplanar Rotation Knee Injury Classification System With Sagittal Plane Translation and Coronal and Axial Rotation Injuries
| Name | Anterolateral Rotation I | Anterolateral Rotation II | Anteromedial Rotation | Posteromedial Rotation I | Posteromedial Rotation II | Posterolateral Rotation |
|---|---|---|---|---|---|---|
| Trauma mechanism |
|
|
|
|
|
|
| Instability (+ secondary instability if advanced) | ALRI (+ AMRI) | ALRI (+ PLRI) | AMRI | PMRI (+ AMRI) | PMRI (+ PLRI) | PLRI |
| Flexion/ extension knee | Flexion to extension | Flexion to extension | Flexion to extension | Extension to flexion | Extension to flexion | Extension to flexion |
| Central or peripheral | Central to peripheral | Central to peripheral | Peripheral to central | Central to peripheral | Central to peripheral | Peripheral to central |
| Order of injury |
|
|
|
|
|
|
| Bone marrow edema/ cartilage impaction | LFC and LTP | LFC (+MFC) and LTP (+MTP) | LFC & LTP | Anterior femur and anterior tibia | Anterior LFC and anterior LTP | Anterior MFC and anterior MTP |
| Medial meniscus | Vertical/ramp | Vertical/ramp | Vertical/ramp | Complex/body | Complex/body | Complex/body |
| Lateral meniscus | Root/LMORT | Root/LMORT | Root/LMORT | Body | Body | Body |
| Potential knee dislocation according to Schenck | KD-1M (step 4) | KD-1L (step 4) | KD-1M (step 4) | KD-1L (step 4) | KD-1M (step 4) | KD-1L (step 3) |
| KD-3M (step 6) | KD-3L (step 5) | KD-3M (step 5) | KD-3L (step 6) | KD-3M (step 6) | KD-1ML (step 6) | |
| KD-4 (step 8) | KD-4 (step 8) | KD-4 (step 7) | KD-4 (step 8) | KD-4 (step 8) | KD-4 (step 7) |
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