Posterior Malleolar Fragment Size as a Predictor of Ligamentous Injuries in Trimalleolar and Quadrimalleolar Ankle Fractures: A Novel Correlative Cohort Study in a South Indian Population

Abstract

Background:

Posterior malleolar (PM) fractures are critical components of complex ankle injuries, particularly in tri- and quadrimalleolar fracture patterns. Although traditionally assessed by fragment size and displacement, emerging evidence suggests a more intricate relationship between PM morphology and associated ligamentous injuries. Aim was to evaluate the association between PM fragment size and concomitant soft tissue injuries, particularly deltoid and syndesmotic involvement in tri- and quadrimalleolar ankle fractures.

Methods:

This ambispective cohort study included 45 adult patients with tri- or quadrimalleolar fractures treated via a posterolateral approach. PM fragments were classified using computed tomography–based systems (Haraguchi, Bartonicek-Rammelt, Mason-Molloy). The presence of ligamentous injuries was assessed intra-operatively and correlated with fragment size. Syndesmotic and deltoid ligament involvement were analysed in relation to PM morphology. Statistical analysis was performed using SPSS version 22.

Results:

Most PM fractures were Haraguchi type 1 (66.7%), Bartonicek-Rammelt type 2 (62.2%), and Mason-Molloy type 2A (60%). Although syndesmotic injury showed no statistically significant association with PM fragment size (P = .135), a novel finding was identified: PM fragments larger than 8 mm were significantly associated with deltoid ligament injuries (P = .01). This correlation suggests a potential predictive role of fragment size in identifying medial-sided instability.

Conclusion:

Posterior malleolar fragment size, particularly fragments exceeding 8 mm may serve as a surrogate marker for deltoid ligament injury in tri- and quadrimalleolar ankle fractures, whereas no significant association was identified with syndesmotic injury. This finding highlights the importance of comprehensive pre-operative imaging and intra-operative assessment to guide fixation strategies and address associated ligamentous pathology.

Level of Evidence:

Level III, evidence from non-experimental descriptive studies.

Introduction

Ankle fractures are among the most common lower extremity injuries, with an estimated annual incidence of 187 per 100 000 adults. ¹ Their distribution is characteristically bimodal, occurring most frequently in young men aged 15-24 years and in elderly women aged 75-84 years. ² Among these injuries, tri- and quadrimalleolar fractures represent particularly complex patterns because of the frequent involvement of the posterior malleolus (PM) and the high likelihood of requiring operative management. ¹⁴ Traditional classification systems, such as the Lauge-Hansen mechanism-based framework, offer limited detail regarding PM morphology. With the increasing reliance on computed tomography (CT), our understanding of PM anatomy and injury patterns has expanded significantly, leading to the development of more sophisticated classification systems, including those proposed by Haraguchi, Bartoníček-Rammelt, and Mason-Molloy. These CT classifications characterise PM morphology as follows: Haraguchi type 1 (posterolateral-oblique) and type 2 (medial extension); Bartoníček-Rammelt type 1 (small posteromedial) and type 2 (posterolateral extending into incisura); Mason-Molloy type 1 (small posteromedial), type 2A (posterolateral involving incisura), and type 2B (larger posterolateral). Quadrimalleolar fractures were defined as trimalleolar equivalents with additional Chaput/Wagstaffe avulsion indicating anterior-inferior tibiofibular ligament (AITFL) injury.

Growing evidence suggests that both the size and morphologic subtype of PM fragments may correlate with associated ligamentous injuries, particularly those involving the syndesmosis and the deltoid ligament. These relationships have important implications for preoperative planning, stability assessment, and fixation strategy. Against this backdrop, the present study was designed to evaluate the association between PM fragment size and concomitant soft tissue injuries in tri- and quadrimalleolar ankle fractures. The primary objectives were to determine whether PM fragment size is associated with syndesmotic disruption and to assess its relationship with deltoid ligament injury.

Materials and Methods

This study was conducted as a 4-year ambispective cohort investigation in a tertiary care hospital. The cohort included 45 adult patients who sustained isolated trimalleolar or quadrimalleolar ankle fractures involving the posterior malleolus and underwent operative fixation through a posterolateral approach. Inclusion criteria were trauma-induced tri- or quadrimalleolar fractures in adults aged 18 years or older with documented ambulatory status prior to injury. Exclusion criteria consisted of pathologic fractures, multiple fractures in the same limb, type 2 diabetes mellitus, and pre-injury non-ambulatory or bedridden status. Ethical clearance was obtained from the institutional review board before commencing the study.

All patients underwent standardised pre-operative imaging, including anteroposterior, lateral, and mortise radiographs, which were classified according to the mechanism-based Lauge-Hansen system. Computed tomography (CT) was performed for all patients to assess PM morphology in greater detail. Fractures were categorised using 3 established CT-based classifications: Haraguchi, Bartoníček-Rammelt and Mason-Molloy. PM fragment size was measured on axial CT slices acquired 1 cm proximal to the tibial plafond to ensure reproducibility. Clinical data were collected through structured assessments and intraoperative findings were meticulously documented, with specific attention to syndesmotic disruption and deltoid ligament injury (Figure 1).

Figure 1.

Illustrative example of the evaluation process for a representative patient.

Ligamentous stability of the distal tibiofibular syndesmosis was assessed intraoperatively prior to PM fixation to avoid potential restoration of stability following fragment stabilisation. Syndesmotic injury was defined as instability involving the anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament, and/or interosseous membrane. Assessment was performed using the Cotton (hook) test, with >2 mm pathologic fibular translation considered positive and external rotation stress testing, with a side-to-side difference >5° indicating instability. The presence of a PM fracture was considered representative of a posterior-inferior tibiofibular ligament (PITFL) avulsion-equivalent injury.

Integrity of the deltoid ligament complex was evaluated in patients with medial malleolar fractures. Deltoid injury was defined by medial clear-space widening >4 mm on preoperative stress radiographs where available, >5 mm on mortise radiographs, or intraoperative evidence of medial instability or ligament disruption during exposure. Ligament-equivalent trimalleolar fracture patterns were included in the study eligibility criteria; however, no such cases were identified in the present cohort. All intraoperative assessments were performed by the study authors under consultant supervision.

Statistical analysis was performed using SPSS version 22 and Microsoft Excel. Categorical variables were summarised as frequencies and percentages and compared using the χ² test or Fisher exact test, as appropriate. Continuous variables were expressed as mean ± SD and analysed using the independent samples t test. A P value <.05 was considered statistically significant, with corresponding 95% confidence intervals reported. A post hoc power analysis was conducted using G*Power software (version 3.1.9.7), which demonstrated a study power of 61% for detecting the observed effect size in the association between deltoid ligament injury and the measured radiographic parameter, supporting the exploratory and hypothesis-generating nature of the study.

Results

The most common age group in the cohort was 50-59 years, comprising 49% (n = 22) of all cases. A slight female predominance was observed, with females accounting for 51% (n = 23) of the study population. Trimalleolar fractures constituted the vast majority of injuries, representing 96% of cases (n = 43), whereas quadrimalleolar fractures were comparatively rare. Supination–external rotation type IV (SER-4), as classified by the Lauge-Hansen system, was the predominant injury mechanism and was identified in 93.3% of patients (n = 42), highlighting the strong association of SER injuries with PM involvement in this cohort. This predominance likely reflects the higher epidemiologic frequency of SER-4 injuries among operatively treated ankle fractures, as the most common fracture pattern, rather than a specific predilection for PM involvement. CT-based morphologic assessment revealed consistent patterns across all 3 classification systems. According to the Haraguchi classification, type 1 (posterolateral oblique) was the most frequently observed variant, present in 67% of cases. The Bartoníček-Rammelt system similarly demonstrated a predominance of type 2 fractures, characterised by a posterolateral fragment extending into the fibular notch, which occurred in 62% of patients. Using the Mason-Molloy classification, type 2A (posterolateral fragment involving the incisura) was the most common subtype, identified in 60% of the cohort. These findings indicate that posterolateral-oriented fracture patterns were consistently the most prevalent across classification systems (Table 1).

Table 1.

Baseline Cohort Characteristics and Posterior Malleolar Fracture Classifications (n = 45).

Characteristics	Predominant Distribution	n (%)
Demographics
Age	50-59 y	22 (49)
Gender	Female	23 (51)
Fracture patterns
Type	Trimalleolar	43 (96)
	Quadrimalleolar	2 (4)
Lauge-Hansen variant	SER-4	42 (93)
CT classification
Haraguchi	Type 1	30 (67)
Bartoníček-Rammelt	Type 2	28 (62)
Mason-Molly	Type 2A	27 (60)

Abbreviations: CT, computed tomography; SER-4, supination–external rotation type IV.

Ligamentous injuries were notable within the cohort. Syndesmotic disruption was identified in 42% of patients (n = 19), with assessment performed prior to PM fixation, whereas deltoid ligament injury was present in 27% (n = 12) (Figure 2). Analysis of PM fragment size (injury absent: 6.68 ± 2.53 mm; injury present: 7.21 ± 2.36 mm) demonstrated no significant association between fragment size and syndesmotic disruption (P = .135). PM fractures may represent avulsion-equivalent injuries of the PITFL, and fixation of the fragment can contribute to restoration of syndesmotic stability. However, a statistically significant correlation was observed between larger fragment sizes and deltoid ligament injury. Fragment sizes exceeding 8.017 mm were strongly associated with deltoid disruption (P = .01). Among deltoid-positive cases, 3 PM fragments demonstrated medial extension toward the deltoid origin on CT, suggesting a possible direct anatomical association, whereas the remaining cases were likely related to indirect injury mechanisms. Fragment size was therefore categorised using a threshold of >8 mm (Figure 3).

Figure 2.

Associated ligamentous injuries in trimalleolar (n = 43) and quadrimalleolar (n = 2) ankle fractures. (A) Syndesmotic injury. (B) Deltoid ligament injury.

Figure 3.

Association between posterior malleolar (PM) fragment size and ligamentous injuries. Image A shows no statistically significant correlation was found between PM fragment size and syndesmotic injury whereas image B shows a significant association between PM fragment size >8 mm and the presence of deltoid ligament injury.

Discussion

The demographic characteristics of our cohort align with prior studies examining PM involvement in ankle fractures, supporting the relevance of our findings within the broader literature. Xie et al ³ and Wang et al ⁴ reported mean presentation ages in the mid-40s to late 40s, with a modest female predominance, reflecting a population in which ligamentous compromise around the ankle is commonly encountered. Our cohort, similarly concentrated in the 50-59-year age group, represents a demographic in which osteoporotic changes in bone and soft tissues may predispose to more complex injury patterns, including syndesmotic and deltoid ligament disruption. The slight female predominance observed in this study further parallels earlier reports and may relate to age-associated reductions in bone density and soft tissue resilience.

The predominance of trimalleolar fractures in our cohort is consistent with the observations of Neumann and Rammelt, ⁵ who similarly noted trimalleolar patterns to be more common than quadrimalleolar injuries. The 96% incidence in our study not only reinforces trimalleolar fractures as the primary complex ankle injury encountered in clinical practice but may also reflect a degree of selection bias toward surgically managed cases at our center. This distribution underscores the need to refine and optimise management strategies for trimalleolar fractures, given their high frequency and substantial contribution to the overall burden of complex ankle trauma. Chaput/Wagstaffe avulsions (AITFL equivalents) were classified as trimalleolar unless clearly separate, explaining low quadrimalleolar rate vs high-volume centers.

The predominance of Lauge-Hansen SER-4 injuries in our cohort (93.3%) is consistent with prior literature, including the findings of Abarquero-Diezhandino et al ⁶ and Lee et al, ⁷ who similarly identified SER-4 as the most frequent mechanism underlying PM fractures. The higher proportion observed in our study likely reflects the selective inclusion of complex fracture patterns requiring operative intervention and CT-based assessment, which inherently enriches the sample for advanced rotational injuries. The markedly lower representation of pronation–external rotation (PER-4) injuries further supports the strong mechanistic predominance of SER-4 in such fractures. Overall, this alignment reinforces the central role of SER-4 as the principal injury mechanism driving complex PM involvement and highlights its relevance in guiding diagnostic evaluation and surgical planning.

CT-based morphologic classifications remain central to understanding PM injury patterns, and the trends observed in our cohort parallel those described in prior foundational work. Within the Haraguchi system, the predominance of type 1 fractures in our patients corresponds with the original series by Haraguchi et al, ⁸ who reported type 1 as the most frequent pattern (67%), as well as with subsequent studies by Morales et al ⁹ (66.6%) and Mitchell et al ¹⁰ (93%). This consistency underscores the stability of the posterolateral-oblique fragment morphology across demographically diverse groups. Similarly, the Bartoníček-Rammelt classification demonstrated a clear concentration of type 2 fragments in our cohort, aligning with the distributions reported by Bartoníček et al ¹¹ (52%) and Morales et al ⁹ (58.3%). These concordant findings emphasise the clinical relevance of the posterolateral fragment extending into the incisura, particularly in relation to syndesmotic involvement and the need for posterior stabilisation. Within the Mason-Molloy framework, our study identified type 2A as the dominant injury morphology (60%), echoing the patterns described by Mason et al ¹² and later corroborated by Morales et al, ⁹ who documented 45.8% type 2A and 39.5% type 2B. Additional support comes from Myatt et al, ¹³ who also reported type 2A as the most prevalent subtype (56.07%). Taken together, the overlap between our findings and those of larger international cohorts reinforces the external validity of these CT-based classifications. More importantly, because each system captures distinct nuances of posterior fragment geometry, their combined use strengthens the ability to correlate fragment size orientation and incisural involvement with the presence of associated ligamentous injuries, an aspect central to the aims of the present study.

This study demonstrated a markedly higher incidence of associated ligamentous injuries, with syndesmotic disruption present in 42% of patients and deltoid ligament injuries in 27%. These values are substantially higher than the 10% syndesmotic involvement reported by van Vlijmen et al. ¹⁴ Clanton et al ¹⁵ further emphasised the strong association between PM fractures and syndesmotic compromise, noting that PM involvement, present in approximately 20% to 45% of all malleolar fractures, serves as a key predictor of distal tibiofibular instability because of the fragment’s integral role in maintaining posterior syndesmotic support. The higher rates observed in the present cohort are likely attributable to the exclusive inclusion of tri- and quadrimalleolar fractures, which constitute inherently more unstable injury patterns with a greater propensity for ligamentous disruption. Biomechanical work by Clanton et al ¹⁵ highlighted the central role of ligamentous integrity in maintaining ankle stability, particularly in the context of multifragmentary fracture configurations where soft tissue structures are subjected to amplified forces. This reinforces the necessity for comprehensive preoperative evaluation, ideally incorporating CT alongside adjunctive modalities such as magnetic resonance imaging (MRI) or focused ultrasonography when clinically indicated, as well as meticulous intraoperative assessment. Inadequate recognition and treatment of these injuries can result in chronic instability, accelerated post-traumatic osteoarthritis, and inferior functional outcomes.

The relationship between PM fragment size and associated ligamentous injury remains insufficiently characterised in current orthopaedic literature, with most studies primarily focusing on syndesmotic disruption while giving limited attention to medial-sided soft tissue injury. Prior work by Gardner et al has shown that larger PM fragments correlate with greater syndesmotic instability, although reported size thresholds have varied considerably. ¹⁶ A recent study by Prasarn et al further emphasised that PM fragments involving more than 25% to 30% of the tibial plafond significantly increase the likelihood of syndesmotic compromise, underscoring the critical role of fragment morphology in predicting instability. ¹⁷ In contrast, the association between PM fragment dimensions and deltoid ligament injury has been far less explored. The present study’s identification of a statistically significant correlation between PM fragment size greater than 8 mm and deltoid ligament injury provides an important contribution by establishing a quantifiable radiographic marker for medial ankle instability. This novel finding carries direct surgical relevance, suggesting that larger posterior fragments should heighten clinical suspicion for deltoid insufficiency and may justify deliberate assessment or repair to restore medial column stability.

Larger PM fragments (>8 mm) associated with deltoid ligament injury may reflect either direct medial extension of the fragment with possible avulsion at the deltoid origin (identified in 3 cases on CT) or an indirect contrecoup mechanism related to SER-IV injuries, wherein loss of PITFL restraint permits unopposed medial stress. The posterolateral surgical approach limits direct assessment of medial soft tissues, and further evaluation with MRI may help clarify these associations. To explain the coexistence of a medial malleolar fracture with a concomitant deltoid ligament rupture, we propose a mechanism based on events occurring during a high-energy SER-4 injury. In high-velocity trauma, the externally applied torque is substantial, generating an initial lateral talar shift with external rotation. This sequence is proposed to first disrupt the AITFL, followed by a fibular fracture as the talus impacts the lateral malleolus. During this phase, the deltoid ligament is subjected to significant tensile strain but typically remains intact. To generate a large posterolateral PM fragment, considerable torque is essential. When this force avulses a substantial large PITFL-bearing fragment, the sudden loss of posterior constraint produces a secondary contrecoup effect, transmitting force from the posterolateral tibia toward the anteromedial ankle. This force vector may contribute to a medial malleolar fracture and simultaneously increase tensile load on the deltoid ligament. Once the tensile stress exceeds its viscoelastic threshold, the deltoid ligament may fail, either immediately before avulsion due to peak torque or immediately after as a consequence of the contrecoup recoil (Figure 4).

Figure 4.

This hypothetical schematic illustrates the Lauge-Hansen SER-4 injury mechanism and the combined occurrence of a medial malleolar fracture and deltoid ligament rupture. Images A, B, and C depict the classic SER sequence, beginning with AITFL rupture followed by lateral malleolar fracture due to externally applied torque and lateral talar shift. Image D demonstrates avulsion of a large posterolateral posterior malleolar fragment, reflecting sudden loss of posterior stability. The resulting contrecoup force transmitted from the posterolateral tibia to the anteromedial ankle produces a medial malleolar fracture and imposes high tensile load on the deltoid ligament. When this tensile demand exceeds the ligament’s capacity, deltoid rupture occurs (E). AITFL, anterior-inferior tibiofibular ligament.

This study has several limitations that warrant consideration. The relatively small sample size and single-center design may restrict the generalizability of the findings to broader populations. Additionally, the absence of preoperative MRI limited the ability to noninvasively assess ligamentous integrity and correlate soft tissue abnormalities with intraoperative findings. These constraints underscore the need for future investigations involving larger, multi-center cohorts and the incorporation of advanced imaging modalities to further validate the observed associations and enhance the predictive accuracy of PM fragment characteristics. No formal sample size or power calculation was performed a priori. A post hoc analysis using G*Power (effect size d = 0.70 for the observed association with deltoid ligament injury) demonstrated a study power of 61%, supporting the exploratory and hypothesis-generating nature of this novel cohort. The 8- mm fragment size threshold was identified through post hoc analysis of the present data set and was not prespecified; prospective validation in an independent cohort is required before this value is applied clinically. Clinical outcome parameters, including functional scores and ankle range of motion, were not evaluated. Deltoid ligament injuries were managed indirectly through fixation of the medial malleolus where present, without suture anchor augmentation or direct ligament repair. Syndesmotic stability was reassessed following PM fixation, and additional syndesmotic stabilisation was performed when residual instability persisted.

Conclusion

This study demonstrates a clinically meaningful association between PM fragment size and concomitant ligamentous injury in tri- and quadrimalleolar ankle fractures. Although syndesmotic disruption did not show a statistically significant relationship with fragment size (P = .135), fragments exceeding 8 mm were significantly associated with deltoid ligament injury (P = .01). In this cohort of 45 patients, this radiographic threshold may assist in preoperative risk stratification and prompt careful intraoperative medial assessment, particularly when the posterolateral approach limits direct visualisation of medial structures. The predominance of posterolateral fragment morphologies across CT-based classification systems further supports the importance of detailed morphological evaluation in understanding injury patterns and informing fixation strategy. Given the substantial incidence of ligamentous compromise in these injuries, comprehensive assessment using appropriate imaging and systematic intraoperative testing may be essential to optimize ankle stability. Larger prospective, multicentre studies incorporating functional outcome measures are required to validate these findings and refine predictive models for ligamentous injury associated with posterior malleolar fractures.

Supplemental Material

sj-pdf-1-fao-10.1177_24730114261445093 – Supplemental material for Posterior Malleolar Fragment Size as a Predictor of Ligamentous Injuries in Trimalleolar and Quadrimalleolar Ankle Fractures: A Novel Correlative Cohort Study in a South Indian Population

Supplemental material, sj-pdf-1-fao-10.1177_24730114261445093 for Posterior Malleolar Fragment Size as a Predictor of Ligamentous Injuries in Trimalleolar and Quadrimalleolar Ankle Fractures: A Novel Correlative Cohort Study in a South Indian Population by Jillella S. R. G. Saran, Shiv M. Ajoy, Rahul Panduranga and Rajesh Simon in Foot & Ankle Orthopaedics