Pathoanatomy and pathomechanics of pertrochanteric fractures – an MRI study

Summary

Background and study aims: Magnetic resonance imaging (MRI) has been used for more than 20 years in the region of the proximal femur to diagnose occult, or incomplete, fractures of the femoral neck and the trochanteric segment. MRI has also potential to contribute to the understanding of the pathogenesis and pathoanatomy of trochanteric fractures.

Methods: The group including 13 patients was examined by MRI for a suspected, or incomplete, fracture of the trochanteric segment within 24 hours post-injury. In all cases, this was the first injury to the hip joint, with the other hip joint remaining intact.

Results: The coronal scans showed a marked fracture line which, in the region of the intertrochanteric line, extended from the base of the greater trochanter (GT) medially and distally and involved the medial cortex. This inclination, however, was gradually changing posteriorwards and close before the posterior cortex. The fracture line was passing vertically along the lateral trochanteric wall as far as the level of the lesser trochanter (LT). Then the fracture line changed its course and ran horizontally to the cortex of the LT. Sagittal scans showed clearly the primary fracture line originating in the greater trochanter, extending medially and starting to separate the posterior cortex.

Conclusion: Analysis of MRI findings has documented that the primary fracture line in pertrochanteric fractures originates in the GT and extends distally, medially and anteriorly towards the anterior cortex, the intertrochanteric line and the LT. Thus, the GT presents a rather vulnerable site and is always broken into more fragments than shown by a radiograph.

Keywords:

MRI – pathoanatomy – pertrochanteric fractures – occult fractures

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INTRODUCTION

Magnetic resonance imaging (MRI) has been used for more than 20 years in the region of the proximal femur to diagnose occult, or incomplete, fractures of the femoral neck and the trochanteric segment. The reliability of this method, including its benefits as compared to CT, has been repeatedly proved by multiple studies. Thus, MRI has helped rationalize the treatment of occult and incomplete trochanteric fractures [1–21].

MRI, however, has yet another, so far only rarely discussed potential, namely to contribute to the understanding of the pathogenesis and pathoanatomy of trochanteric fractures. Classical classifications of trochanteric fractures based on radiographs, including the AO classification [22,23], have been questioned by recent CT studies in a number of aspects [24–29]. As compared to CT examination, MRI is apparently much more sensitive when capturing the initial phase of trochanteric fractures, which may help to gain a better insight into their pathomechanism and pathoanatomy.

The aim of the study was to describe the onset and progression of fracture lines which correlate with autopsy findings [30].

MATERIAL AND METHOD

Material

The study group included 13 patients examined by MRI for a suspected, or incomplete, fracture of the trochanteric segment in the period between January 2012 and December 2021. In all cases, this was the first injury to the hip joint, with the other hip joint remaining intact. The patients were skeletally mature, without radiological signs of osteoarthritic changes, and none of them suffered from a systemic rheumatic disease. Each patient reported acute trauma of varying intensity (sudden excessive load of the extremity, fall when walking, etc.). The cohort comprised 7 men and 6 women, all of them, but for one exception, older than 57 years. The mean age of these 12 patients was 76 years, and one patient was 23 years old (Tab. 1).

Method

A radiograph of the pelvis and ap and axial radiographs of the affected hip were obtained in all patients. In 6 patients, a radiograph revealed a fracture of the greater trochanter, with a marked displacement only in 1 case. In the other 7 patients, fracture of the greater trochanter was suspected. In 2 patients with a suspected fracture of the greater trochanter, a radiograph showed an incomplete fracture line, in one case in the ap and in the other case only in the axial views. In 4 patients, a suspected fracture line was revealed in an ap view.

MRI examination was performed in all 13 patients within 24 hours post-injury. We used 2 Tesla MRI scanners (Gyrex Prestige, Elscint) to obtain 4 mm thick coronal, sagittal, axial T1 weighted images and coronal T2 weighted images.

Assessment

The obtained images allowed to identify fracture lines, assess their course and nature, identify individual fragments and thus get a picture of the fracture mechanism. All findings were consulted with an experienced musculoskeletal radiologist who was not part of the authors’ team.

 

 

Fig. 1. / Obr. 1.

Course change of complete fracture line in anterior-posterior direction. Red arrow fragment of greater trochanter.
ANT – anterior scan, POST – posterior scan

Změna průběhu kompletní lomné linie v předo-zadní směru. Červená šipka – fragment velkého trochanteru.
ANT – přední sken, POST – zadní sken

 

 

Fig. 2. / Obr. 2.

Course of incomplete fracture line in anterior-posterior and lateral-medial directions, with the most marked changes seen in the region of greater trochanter. Anteriorwards, the fracture line is gradually disappearing. Red arrow – intact posterior cortex, white arrows – fragments of greater trochanter.

ANT – anterior scan, POST – posterior scan, LAT – lateral scan, MED – medial scan

Průběh inkompletní lomné linie v předo-zadním a lateromediálním směru, nejmarkatnější změny jsou patrné v oblasti velkého trochanteru. Směrem vpřed lomná linie postupně mizí. Červená šipka – intaktní zadní kortikalis, bílá šipka – úlomky velkého trochanteru.
ANT – přední sken, POST – zadní sken, LAT – laterální sken, MED – mediální sken

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Fig. 3. / Obr. 3.

Changes in characteristic of the fracture line in anterior-posterior direction. A well-defined, narrow, almost straight fracture line in anterior cortex; fracture line almost absent on middle scans; gradually appearing, wide and irregular fracture line on posterior scans.
ANT – anterior scan, POST – posterior scan

Změna charakteru lomné linie v předo-zadním směru. Dobře patrná, úzká, téměř rovná lomná linie v přední kortikalis; ve střední části lomná linie téměř mizí, postupně se zjevující široká a nepravidelná lomná linie na zadních skenech.
ANT – přední sken, POST – zadní sken

 

 

Tab. 1. Basic characteristics of the study group.
Tab. 1. Základní charakteristika souboru.

No.	Age (years)	Gender	X-ray GT Fx	MRI GT Fx	X-ray Fx Line	MRI Fx Line
1	83	F	suspected	visible	nonvisible	incomplete
2	85	F	suspected	visible	nonvisible	incomplete
3	86	F	suspected	visible	nonvisible	incomplete
4	74	F	suspected	visible	suspected	incomplete
5	72	F	nondisplaced	visible	nonvisible	incomplete
6	66	M	nondisplaced	visible	nonvisible	incomplete
7	57	M	nondisplaced	visible	nonvisible	incomplete
8	76	M	nondisplaced	visible	nonvisible	incomplete
9	84	F	displaced	visible	suspected	incomplete
10	79	M	suspected	visible	suspected	complete
11	66	M	nondisplaced	visible	suspected	complete
12	86	M	nondisplaced	visible	axial view	complete
13	23	M	nondisplaced	visible	ap view	complete

F – female, M – male, X-ray – GT Fx – fracture of greater trochanter visible on radiograph, MRI – GT Fx – fracture of greater trochanter visible on magnetic resonance imaging, X-ray – Fx Line –fracture line visible on radiograph, MRI – Fx Line – form of fracture line on MRI scans

F – ženy, M – muži, X-ray – GT Fx – zlomenina velkého trochanteru viditelná na radiografu, MRI – GT Fx – zlomenina velkého trochanteru viditelná na magnetické rezonanci, X-ray – Fx Line – lomná linie viditelná na radiografu, MRI – Fx Line – rozsah lomné linie na MR skenech

 

 

RESULTS

Fracture line in the coronal plane

The coronal scans showed a marked fracture line which, in the region of the intertrochanteric line (anterior cortex), extended from the base of the greater trochanter (GT) obliquely in about 45-degree angle medially and distally and involved the medial cortex. This inclination, however, was gradually changing posteriorwards and close before the posterior cortex the fracture line was passing vertically along the lateral trochanteric wall as far as the level of the lesser trochanter (LT). Then the fracture line changed its course and ran horizontally to the cortex of the LT, resembling the letter “L” by its shape (Fig. 1). This finding was detected in 4 patients with a complete fracture. In patients with an incomplete fracture, the fracture line started similarly, but did not involve the medial cortex. Posterior scans showed a fracture line extending more distally, gradually getting shorter and less marked anteriorwards (Fig. 2).

The nature of the fracture line was also changing. In all complete fractures, the fracture line in the region of the anterior cortex was fine, narrow and sharp. In consecutive scans of complete fractures, the fracture line became less visible in the middle third of the proximal femur (Fig. 3). In incomplete fractures, the fracture line was missing or incomplete in the anterior part. However, it was of a similar nature as in complete fractures. In the posterior third, the fracture line was well seen, widened and irregular in all 13 cases (Fig. 4).

Fracture line in the sagittal plane

Sagittal scans showed clearly the primary fracture line originating in the greater trochanter, extending medially and starting to separate the posterior cortex, i.e., the future posterior fragment (Fig. 2). In 2 complete fractures and 1 incomplete fracture, the posterior cortex was completely disrupted (Fig. 5).

Fractures of the greater trochanter

This injury was found in all 13 cases. MRI revealed that GT was broken into multiple parts regardless of bone quality (Ptn 13), including 6 cases when a radiograph showed it as a solid fragment. Axial scans demonstrated that some GT fragments were pulled by short external rotators medially.

DISCUSSION

Our findings may be interpreted as follows: the fracture line always originated in GT and extended simultaneously distally, anteriorly and medially. This was indicated by absence of the fracture line in the region of the anterior cortex in incomplete fractures. A complete fracture resulted only from disruption of the anterior and medial cortices. The posterior cortex could be disrupted already during propagation of the fracture line, but in complete fractures, it might remain intact.

A different nature and course of the fracture lines in anterior and posterior coronal scans can be explained by the injury mechanism. Pertrochanteric fractures are commonly caused by external rotation and varus forces. External rotation first of all brings about trabecular compression of the cancellous bone in the region of the fossa trochanterica (a wide irregular fracture line), when the femoral neck base is forced into the trochanteric segment. At the same time, tension forces act on the anterior cortex producing a thin, sharp fracture line (Fig. 6). The axis of this rotational movement is obviously situated in the center of the trochanteric segment. This is also indicated by the fact that in complete fractures, the fracture line in the central part can only be seen minimally in coronal scans. As a result, the central part would be the transitional zone where tensile forces turn into compression forces. This hypothesis was confirmed also by the analysis of postmortem specimens of a patient with a pertrochanteric fracture, where the macroscopic character of fracture lines corresponded to MRI findings (Fig. 7) [30]. Another interesting fact is that GT was broken into multiple fragments in all cases, including a young, 23-year-old patient. This finding corresponds to the results of recent CT studies dealing with pathoanatomy of trochanteric fractures [2,3,9,10,17,20]. A certain role in this context is played by the pull of short external rotators as well as by the fact that some of the GT areas are not reinforced by muscle attachments. We have not found a similar interpretation of MRI findings in occult and incomplete trochanteric fractures in the literature. Nevertheless, the same course, propagation and character of fracture lines can be seen in images published by a number of authors [16,17,19–21]. Other authors have proved that the so called isolated fracture of GT is actually an incomplete trochanteric fracture [4,10–12,16,17,19,20].

All this is equivalent to our findings.

The fact that in all our cases and also in other studies, the fracture line originated in GT and extended medially and distally, questions the existence of the so-called three-part pertrochanteric fracture (AO 31A2.3), where the third fragment is formed by the LT only, and the GT remains intact [23,30].

CONCLUSION

The analysis of MRI findings has documented that the primary fracture line in pertrochanteric fractures originates in the GT and extends distally, medially and anteriorly towards the anterior cortex, the intertrochanteric line and the LT. Thus, the GT presents a rather vulnerable site and is always broken, usually into more fragments than shown by a radiograph or CT. This study contributes to a better understanding of pathoanatomy of pertrochanteric fractures and, at the same time, it questions the existence of certain types presented in the AO classification (31A2.3).

 

 

Fig. 4. / Obr. 4.

Fracture line on posterior scans. A) radiograph of displaced fracture of greater trochanter, incomplete fracture line is visible; B) vertical broad incomplete fracture line on T2 – weighted scan; C) fragment of greater trochanter is split into several pieces on T1 – weighted scan.

Lomná linie na zadních skenech. A) RTG snímek dislokované zlomeniny velkého trochanteru, inkompletní lomná linie je dobře patrná; B) vertikální, široká, inkompletní lomná linie na T2 – vážených skenech; C) fragment velkého trochanteru rozlomený do několika částí na T1 – vážených skenech.

 

 

Fig. 5. / Obr. 5.

Fracture of posterior cortex in incomplete pertrochanteric fracture. ANT – anterior scan, POST – posterior scan, LAT – lateral scan, MED – medial scan, red arrows – fracture posterior cortex

Zlomenina zadní kortikalis u inkompletní pertrochanterické zlomeniny. ANT – přední sken, POST – zadní sken, LAT – laterální sken, MED – mediální sken, červené šipky – zlomenina zadní kortikalis

 

 

Fig. 6. / Obr. 6.

Reduced postmortem specimen of three-part pertrochanteric fracture. Fracture line of anterior cortex is narrow and regular, posterior fracture line is wide, irregular and defective and indicates trabecular compression.

Postmortem reponovaná třífragmentová pertrochantrická zlomenina. Lomná linie v přední kortokalis je úzká a pravidelná, zadní lomná linie je široká, nepravidelná a svědčí o kompresi spongiózy.

 

 

Fig. 7. / Obr. 7.

Pathomechanism of pertrochanteric fracture. External rotation of leg generates tension forces on anterior cortex and compression forces on posterior cortex.

Pathomechanizmus pertrochanterické zlomeniny. Zevní rotace dolní končetiny vytváří tenzní síly působící na přední kortikalis a kompresní síly působící na zadní kortikalis.

 

 

Acknowledgements

The authors wish to thank Prof. Chris Colton, MD, FRCS, and Ludmila Bébarová, PhD for their assistance in the editing of the manuscript.

Funding

This study was supported by IP DZRVO MO 1012.

Conflict of interests

The authors declare that they have no conflict of interest related to the creation of this article, and that this article has not been published in any other journal with access to congress abstracts.