Anterior translation of the tibia in relation to femur in mucoid degeneration of ACL – An observational study

Abstract

Background

Mucoid degeneration (MD) of the anterior cruciate ligament (ACL) are a well-known pathological entity.

We have encountered several patients with MD of the ACL, found to have a anterior translation of tibia a exceeding 5 mm with an intact ACL. We studied this cohort and investigated the likely cause of this.

Methods

A retrospective search of our department's radiology system to identify all patients referred from the knee orthopaedic clinic for MR imaging over a span of 10 years. All patients had MD within the substance of the ACL and an intact ACL. We evaluated the degree of anterior translation of the tibia (ATT) in relation to the femur in mucoid degeneration of ACL.

Results

We identified 464 consecutive cases. The mean age was 52 years. There was a male predominance of 261 to 203 female. The average PTF measurement was 2.4 mm with a range of 0-20mm. Of the 464 cases, 3 397 patients had an insignificant ATT of < 5 mm. (0mm. 67. 67 67 patients had a ATT >5 mm. Of them, 32.8% had a ATT of 6 mm, 53.7% had a ATT range between 7 and 9 mm, with the remaining 13.4% above 9 mm in length.

Conclusion

It is essential to look for other secondary signs of ACL tears and not only focus on ATT as well as correlate this with clinical findings.

1. Introduction

Mucoid degeneration (MD) of the anterior cruciate ligament (ACL) is a well-known pathological entity, characterised by distinct infiltration of the ACL fibres with a mucoid like substance.1, 2, 3 Although the pathogenesis is not well understood; it is hypothesized that MD may lead to the formation of ACL ganglionic cysts (GC), and the presence of these two lesions may coincide.⁴ Various risk factors have been proposed as being responsible for the formation of these lesions, including previous ligamentous injury, degeneration in the elderly population or even congenital anomalies.⁵ Clinically, they are usually asymptomatic and incidentally found on MRI or arthroscopy performed for patients with non-specific knee pain and limitations in the normal range of motion of their knee joints.⁶

MRI remains the gold standard for investigation and diagnosis of MD, and although straightforward, it is important not to confuse this with complete or partial thickness tears of the ACL.⁷ To increase diagnostic confidence of diagnosing ACL tears, many radiologists focus on secondary signs that are associated with this injury. The anterior translation of tibia in relation to the femur (ATT) is a reliable secondary sign of ACL tear which is akin to the clinical anterior drawer test.⁸

In our tertiary orthopaedic centre, we have encountered several cases of MD of the ACL with anterior translation of the tibia of more than 5 mm and an intact ACL. We performed a retrospective study to review the incidence/prevalence of the ATT in these patients.

2. Methods

Local institutional board approval was obtained for this study. We performed a retrospective search of our department's radiology system for the word “mucoid’’ in the reports of MRI of knees performed between 2008 and 2018. As an elective orthopaedic institution, our care does not commonly include acute knee/ACL injuries.

The inclusion criteria consisted of all patients with MD found within the substance of the ACL and an intact ACL (i.e. no evidence of partial or full tear). The criteria of MD of ACL was poor definition of the two bundles of ACL with intact fibres (anteromedial and posterolateral bundles). Using the sagittal view of the fat suppressed proton density sequence of MRI, at the level of the mid lateral femoral condyle, we evaluated the measurement of the degree of ATT. This was performed by drawing two vertical lines’ one at the level of the posterior contour of the lateral tibial condyle and the other on the posterior part of the lateral femoral condyle. The distance between these two lines were measured⁹ (Fig. 1). All images were reviewed and ATT calculated by an experienced consultant musculoskeletal radiologist (experience of more than 10 years) within our department. Two further readers (less than a year experience) also measured a proportion of these cases for interobserver reliability (kappa 0.71). MR was performed on Seimens Skyra 3T (Siemens Healthcare, Erlangen, Germany).

Fig. 1.

Diagrammatic representation of measurement of anterior translation of tibia in relation to femur. Distance between vertical line along the posterior tibial condyle (red line) and posterior femoral condyle (dotted line).

3. Results

We identified 464 consecutive cases in our radiology database that fulfilled the inclusion criteria. Post-surgical cases were excluded. The mean age of our patients was 52 years (14–90 years) and there was a male predominance of 261 to 203 females (ratio of 1.3:1). The average ATT was 2.4 mm with a range of 0–20 mm. Of the 464 cases, 397 patients (85.6%) had an insignificant ATT of < 5 mm. The bulk of them, 205 patients, had no ATT (0 mm). 192 (41.3%) cases had a ATT range between 1 and 5 mm (Table 1). 67 (14.4%) patients had a ATT >5 mm. Of them, 22 (32.8%) had a ATT of 6 mm, 36 (53.7%) had a ATT range between 7 and 9 mm, with the remaining 9 cases (13.4%) above 9 mm in length (Fig. 2, Fig. 3).

Table 1.

Table showing the ATT measurements within our Cohort (including percentages).

Anterior Translation of tibia Measurement (mm)	Number of Patients (n)	Percentage of Total %
0	205	44.2
1	32	6.9
2	48	10.3
3	39	8.4
4	38	8.2
5	35	7.5
6	22	4.7
7	19	4.1
8	12	2.6
9	5	1.1
> 9	9	1.9
Total	464

Fig. 2.

Sagittal Proton density fat suppressed images showing ACL ganglion (a) and anterior translation of tibia in relation to femur (b).

Fig. 3.

Sagittal Proton density fat suppressed images showing mucoid degeneration of ACL (a) and anterior translation of tibia in relation to femur (b).

4. Discussion

The anterior cruciate ligament is one of the two cruciform ligaments that are important in providing stability of the knee joint. It originates from the anterior and medial aspect of the intercondylar region of the tibial plateau and travels superiorly, laterally, and posteriorly to insert into the lateral femoral condyle.¹⁰ The ACL is unique in its structure; it is cost commonly composed of 2 bundles (the anteromedial and posterolateral) that provide approximately 85% of the total force in preventing anterior translation of the tibia in relation to the femur.¹¹ Additionally, the ACL counteracts excessive forward, and backward motion of the tibia during flexion and extension, as well as providing stability during valgus and varus stresses in the knee joint.^11,12

ACL mucoid degeneration is not an uncommon pathology described in the literature13, 14, 15, 16, that is present in approximately 10% of examinations (MRI).¹ It was first described by Kumar et al., in 1999.¹⁷ ACL MD can affect patients of all ages, but are most commonly found in the elderly population with degenerative changes of the knee. Our study revealed a mean age of 52 years within our cohort, closely correlating with previous studies in the literature.18, 19, 20

The pathogenesis of MD is not well understood and several theories have been proposed to account for its development, such as the interposition of synovial fluid within the substance of the ACL (synovial theory),²¹ previous history of trauma (traumatic theory)²² or simply due to ageing (degenerative theory).¹⁴

ACL tears, whether partial or complete can sometimes be confused with ACL mucoid degeneration. MR imaging is the gold standard for diagnosis of both pathologies. The characteristics of MD on MRI include: a hazy thickened ligament with increased signal intensity on T2WI and a classical ‘celery stalk sign.’^23,24 Despite this, ACL tears can be confused with mucoid degeneration leading to misdiagnosis. Radiologists may therefore, assess for secondary signs of ACL injury in cases of mucoid degeneration, which are thought usually to be absent. These secondary signs may include bone bruising, meniscal tears, or posterior translation of the femur.

Anterior translation of tibia in relation to femur is a reliable sign used to diagnose ACL tears and demonstrates a diagnostic sensitivity of 86% and specificity of 99% respectively.⁸ ATT of more that 5 mm is highly suggestive of an ACL tear, while more than 7 mm is considered diagnostic.²⁵ As far as we are aware, there has been no previous study to show the correlation between posterior translation and the presence of mucoid degeneration in an intact ACL. In our study, 14.4% of patients had a ATT of more than 5 mm in length with normal integrity of the ACL. It is therefore important for those interpreting knee MRI to be aware that both ACL injury and MD can lead to ATT (Fig. 2, Fig. 3). A combination of MRI features needs to be used for accurate diagnosis of both entities.

From this study, it is difficult to interpret the reasons why patients with MD have an increased ATT. It is a well-known phenomenon that MD and ganglionic cysts are capable of increasing and decreasing in size.^26,27 The ACL, like any other ligament in the body should normally be taught during extension. We hypothesize that these lesions, when interposed between the fibres of the ACL, can increase and decrease with time. When they regress, this may lead to laxity of the ACL, which may result in increased ATT (Fig. 4). However, this theory is not without its limitations, as a series of MRI assessments are required to follow up on the change in structure of these lesions.

Fig. 4.

Diagrammatic representation showing normal ACL(a), ACL with intrasubstance ganglion and mucoid degeneration (blue) (b and c).

4.1. Limitations

This study was retrospective and no arthroscopic correlation has been assessed, as the diagnosis of MD was solely a radiological diagnosis.

5. Conclusion

Increased ATT is seen in 14.4% of patients with MD. One needs to be aware of this and use primary and other secondary signs in order to diagnose ACL tears.

Funding

No funding.

Declaration of competing interest

None.