Abstract
Purpose
To evaluate the correlation between trochlear dysplasia and acetabular coverage.
Materials and methods
109 retrospective CT studies referred from the young adult knee clinic were independently reviewed by two observers. Anterior acetabular (AASA) and posterior acetabular (PASA) sector angles were calculated bilaterally on axial CT. Trochlear dysplasia was graded using the Dejour classification (A-D). ANOVA test was used.
Results
Dejour types A, B and D trochlear dysplasia were associated with a significantly increased AASA (P value = 0.0011).
Conclusion
Our results demonstrate a significant relationship between trochlear dysplasia and anterior acetabular coverage, as measured by AASA.
Keywords: Acetabular, Morphology, Trochlear dysplasia
1. Objective
Trochlear dysplasia is common in children but is also found in adolescents and young adults with no prior history of childhood knee problems. In our institution, we have observed that many patients with trochlear dysplasia also have varying degrees of acetabular dysplasia, specifically acetabular over coverage. Abnormalities of the hip joint affect the growth plates of the distal femur, hence it is plausible to assume a relationship between the biomechanics of the knee and hip. The aim of our study was to evaluate the correlation between trochlear dysplasia and acetabular coverage, as measured by the anterior acetabular (AASA) and posterior acetabular (PASA) sector angles.
2. Introduction
Trochlear dysplasia is known to be an important cause of patellar instability, and predisposes patients to recurrent patella dislocations.1 The relationship between the patello-femoral joint and the hip joint is not well established in the current literature. The femur directly connects the hip and knee joint, thus it is plausible to assume pathologies affecting one joint will have a direct/indirect effect on the other.
Abnormal bony morphology of the hip is often accompanied by variations in the thickness of the underlying articular cartilage. Hip dysplasia is characterised by reduced acetabular cartilage coverage, which potentially causes increased stress within the hip joint.2 To compensate or as a result of this, the patient may alter the local geometry/rotational profile of the acetabulum. We postulate that this could potentially offset the patello-femoral joint, leading to alterations in trochlear morphology.
At our institution, we routinely examine the images of young patients with varying degrees of trochlear dysplasia. We have noticed an ever-increasing number of this particular patient group also having abnormal acetabular morphology, usually over coverage. From our search, there are currently no articles in orthopaedic literature discussing the relationship between acetabular coverage and trochlear dysplasia.
3. Acetabular coverage
Excessive acetabular coverage typically exhibits itself as partial or complete osseous metaplasia.3 This can lead to increased contact and friction between the femoral head-neck junction and acetabular rim. This can lead to damage of the underlying acetabular cartilage resulting in labral tears and eventually arthritis of the hip joint. This repeated contact between the enlarged acetabular rim and proximal femur has been termed pincer type femoro acetabular impingement (FAI).3
On CT, acetabular coverage can be measured by using the anterior and posterior acetabular sector angles, AASA and PASA respectively (Fig. 1).4 On the axial CT, at the slice of maximum sphericity of the femoral heads, a line is formed by joining the centre of the femoral heads. AASA is the angle between this and line joining the anterior edge of the anterior column of acetabulum and centre of femoral head and PASA is angle between the line joining the centre of head to posterior edge of the posterior column of acetabulum with the line joining the centre of both femoral heads. It is already known that reduced anterior and posterior acetabular support ie-decreased AASA (less than 50°) and PASA (less than 90°) is associated with hip dysplasia.5
Fig. 1.
(diagrammatic representation) and (corresponding CT image). AASA is the angle between the anterior acetabular margin (B), the centre of the femoral head, and the intercapital centre line (A). PASA is the angle between the posterior acetabular margin (C), the centre of the femoral head and the intercapital centre line (A). 50–60° is usually taken as normal for the AASA, 90–100° for PASA.
4. Trochlear dysplasia
Abnormal morphology of the trochlear groove is termed trochlear dysplasia. It is commonly seen in young patients with recurrent patella dislocation. Henri Dejour described this as one of four factors, and possibly the most important factor which can lead to patellar instability.6 The defining characteristic of trochlear dysplasia is a shallow and flattened trochlear groove. This can be quantitatively defined as an increased sulcus angle of more than 145°. Abnormal trochlear morphology has been formally classified into four types by Dejour et al. (Fig. 2).6
Fig. 2.
The four types of trochlear groove dysplastic morphology as described by Dejour et al.: diagrammatic representation and CT image.
N- Normal.
Type A- Normal trochlear groove shape, but with a shallow sulcus angle more than 145°.
Type B- Flattened trochlear groove.
Type C- High lateral trochlear facet with a hypoplastic medial facet. Type D- Features of type C along with a prominent convex bone protrusion-so called 'cliff' pattern.
Type A trochlear dysplasia has a shallow sulcus and Type B has a falt sulcus. The medial facet of trochlea is hypoplastic and high lateral facet in type C and the trochlea is cliff shaped in type D. Currently, there is no consensus as to the exact cause of trochlear dysplasia. It is known that the cartilaginous trochlear angle is already formed at birth but the bony trochlea gains shape and depth during adolescence.7 It is unclear if a genetic component, mal-tracking, or unbalanced forces causing trochlear remodelling in infancy plays a role in causing dysplasia. Current research suggests that all of the aforementioned components can remodel the trochlear groove in childhood.7
5. Materials and Methods
Local ethical committee approval was obtained. Using our departments radiology information system, we performed a search to identify patients referred for computed tomography (CTs) of the lower limbs (rotational profile) over a 5-year period (2013–2018). Rotational profile of CT involved 3 axial blocks of 3 mm slices of hip, knee and ankle joints. All the patients who were referred from the young adult knee orthopaedic clinic without any previous history of surgery were included in this study. Patients with trauma and infection were excluded.
In this series of 125 consecutive patients, 16 were excluded due to having a hip prosthesis insitu or severe acetabular dysplasia. The CT slices were reconstructed in soft tissue and bone windows with a thickness of 1 mm. (64 slice CT scanner, Siemens Healthcare, Erlangen, Germany).
CT studies were independently reviewed by one consultant musculoskeletal radiologist with more than 8 years experience and one senior radiology registrar. AASA and PASA were calculated bilaterally on axial CT imaging as described earlier (Fig. 1). Trochlear dysplasia was graded qualitatively using the Dejour classification if present (Fig. 2).
6. Statistical analysis
5 groups were analysed-normal (ie no trochlear dysplasia) and patients within Dejour types A-D. Using the AASA and PASA obtained for each group we performed a comparative analysis using the analysis of variance test (ANOVA). Intra- and inter-observer variability was calculated. A p value less than 0.05 was considered to indicate a statistically significant difference.
7. Results
Of the 109 patients, 73 were female, 36 male with a mean age of 28 years. The results of AASA and PASA with corresponding p values are shown in Table 1.
Table 1.
Table 1 shows how the anterior (AASA) and posterior (PASA) angles vary with increasing severity of trochlear dysplasia (Groups A-D). AASA and PASA was also measured for a normal group of patients ie-no trochlear dysplasia. Corresponding p values are shown.
| Dejour classification group | AASA (normal 50–60°) | PASA (normal 90–100°) |
|---|---|---|
| A | 65.4 | 95.7 |
| B | 61.9 | 94.5 |
| C | 57.8 | 96.8 |
| D | 63 | 94.5 |
| Normal | 60.3 | 98.5 |
| P value | 0.0011 | 0.4924 |
Dejour types A,B and D trochlear dysplasia were associated with a significantly increased AASA when compared to the control group of normal trochlear anatomy (p value = 0.0011). The AASA value was not raised in the Dejour type C group. No statistically significant correlation between trochlear dysplasia and PASA was found (P value = 0.4924). The intra- and inter-observer variability over 20 cases was good to excellent.
8. Discussion
Our results show that the AASA was increased (acetabular over coverage) in trochlear dysplasia types A,B and D and was statistically significant. No general trend between an increasing AASA and severity of trochlear dysplasia was seen.
An increased AASA exhibits as increased acetabular bony coverage, this can eventually lead to pincer type FAI. Such patients tend to clinically exhibit reduced range of motion in hip flexion, internal rotation, and/or adduction.8 Pincer type FAI is aggravated by movements requiring excessive hip flexion. When this motion is repeated over time there is maximal and repeated contact between the excess acetabular bony coverage and the neck of the femur.8 This abnormal high force can in theory be transmitted down the femur and affect the trochlear groove. Over time, this may lead to varying degrees of trochlear dysplasia. It is unclear whether the changes in the hip or knee may have happened first and exactly how one may affect the other.
There is now a growing body of literature linking anterior knee pain with hip abnormalities. Alfonso and co-workers reported that CAM type impingement clinically produced an external rotational force which maybe responsible for producing anterior knee pain.9 7 patients with anterior knee pain had CAM type impingement refractive to conservative treatment. Both the hip and knee pain reduced significantly following hip surgery.9 Although not specifically studied, it is therefore plausible to assume that the abnormal force generated in hip flexion in pincer type FAI patients may also cause knee pain. However, whether this directly affects the trochlear groove remains to be proven. Yildirim and co-workers also observed that an exertional rotational femur deformity more than 10° brings on anterior knee pain.10
The main limitation of our study is the lack of correlation between imaging findings with clinical symptoms of hip impingement. Also, we are unable to explain why the AASA was not raised in trochlear dysplasia type C patients. Larger scale studies examining any direct causal relationship are required to draw any definitive conclusion. This together with clinical examination and long term follow up will be mandatory to further confirm our results.
Despite this, we feel our findings may have potentially important clinical implications for patients with trochlear dysplasia. The dynamics of the patients gait or presence of knee pain may not be resolved without treating any underlying acetabular pathology or vice versa. Our data suggests that assessment of FAI should be considered when examining patients with anterior knee pain or have a history of patellofemoral instability. This should especially be the case in patients who are refractory to conservative treatment.
Surgery for trochlear dysplasia and pincer type FAI is well established. If a causative link on imaging is seen and there are clinical symptoms at both sites, consideration should be given to managing both the acetabular and trochlear abnormalities, especially if conservative measures have failed.
9. Conclusion
Our results demonstrate a significant relationship between trochlear dysplasia and anterior acetabular coverage, as measured by the AASA. This could predispose to pincer type FAI. It maybe that the altered biomechanics and abnormal forces produced in pincer type FAI can lead to trochlear dysplasia or vice versa. We propose routine CT analysis of the knees for trochlear dysplasia as part of the imaging protocol for patients with acetabular dysplasia.
Conflicts of interest
The authors declare that there are no conflicts of interest.
Financial disclosures
No.
References
- 1.Zaffagnini S. The patellofemoral joint: from dysplasia to dislocation. EFORT Open Rev. 2017;2 doi: 10.1302/2058-5241.2.160081. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Ashwell Z.R. Lateral acetabular coverage as a predictor of femoroacetabular cartilage thickness. J Hip Preserv Surg. 2016;3(4):262–269. doi: 10.1093/jhps/hnw034. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Hayes M.H., Royer N.K. The relationship of acetabular dysplasia and femoroacetabular impingement in hip osetoarthritis: a focussed review. PMR. 2011;3(11):1055–1067. doi: 10.1016/j.pmrj.2011.08.533. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Anda S. Acetabular angles and femoral anteversion in dysplastic hips in adults: CT investigation. J Comput Assist Tomogr. 1991;15:115–120. doi: 10.1097/00004728-199101000-00018. [DOI] [PubMed] [Google Scholar]
- 5.Akiyama M. Femoral anteversion is correlated with acetabular version and coverage in Asian women with anterior and global deficient subgroups of hip dysplasia: a CT study. Skeletal Radiol. 2012;41:1411–1418. doi: 10.1007/s00256-012-1368-7. [DOI] [PubMed] [Google Scholar]
- 6.Dejour D. Eassai de classification. Med Hyg. 1998;56:1466–1471. [Google Scholar]
- 7.Nietosvaara Y., Aalto K. The cartilaginous femoral sulcus in children with patellar dislocation: an ultrasonographic study. J Pediatr Orthop. 1997;17(1):50–53. [PubMed] [Google Scholar]
- 8.Banerjee P., Mclean C.R. Femoroacetabualr impingement: a review of diagnosis and management. Curr Rev Musculoskelet Med. 2011;4(1):23–32. doi: 10.1007/s12178-011-9073-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Sanchis-Alfonso V. Cam Femorocacetabular impingement as a possible explanation of recalacitrant anterior knee pain. Case Rep Orthop. 2016 doi: 10.1155/2016/2064894. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Yildrim A.O. The effect of rotational deformity on patellofemoral parameters following treatment of femoral shaft fracture. Arch Orthop Trauma Surg. 2013;133(5):641–648. doi: 10.1007/s00402-013-1705-x. [DOI] [PubMed] [Google Scholar]