In order to standardize and facilitate the understanding of the location, size, and configuration of the femoral condyle fracture, Letenneur et al^[13] proposed a classification system for Busch-Hoffa fractures, considering the fragment size and orientation of the fracture line. Type I is a pattern whose fracture line extends towards the posterior cortex of the distal metaphysis of the femur; type II is an entirely intra-articular (osteochondral) fracture, which can be subdivided into A, B, or C according to the fragment size (A-75% of the condyle size, B-50% and C-25%); type III is characterized by an oblique fracture line, being therefore a more stable pattern.^[13–15] Analyzing types I, II, and III, the type II presents smaller size, less blood supply and, theoretically, a greater probability of osteonecrosis and nonunion, although this fact has not been proven in the literature.^[16]

Figure 2 illustrates the types of fracture according to the Letenneur classification. Pires et al^[17] proposed a modification to the original Letenneur classification, adding a variant pattern characterized by the presence of a type I fracture associated with a comminution zone or an intercalary fragment, which produces joint depression in the weight bearing zone of the femoral condyle. This modification of the original classification is supported by the mapping study of 74 distal fractures of the femur in the coronal plane carried out by Xie et al.^[5] The authors reported that central comminution (intercalary fragment) was present in 44.9% of coronal fractures of the lateral condyle. The authors also demonstrated that lateral fractures, in addition to being more frequent, have a more vertical fracture line and are more concentrated in the weight bearing zone of the femoral condyle than medial fractures.^[5] Richards et al,^[18] in a retrospective analysis of 55 CT scans of patients with intercondylar fractures of the distal femur, observed that 26 patients (47%) presented no coronal plane fracture line, 6 (11%) presented a medial coronal plane fracture, 15 (27%) presented a lateral coronal plane fracture, and 8 (15%) presented bicondylar coronal plane fractures. The authors identified several major fracture fragments, as described: anteromedial, posteromedial, anterolateral, and posterolateral fracture fragments.^[18] Intercondylar comminution as well as medial and lateral central condylar comminution at load bearing zone of the condyles were frequently observed. Orapiriyakul et al,^[4] in their case series, reported that the lateral fractures were more common and more complex than the medial ones.

As above mentioned, lateral Busch-Hoffa fractures present different morphology and characteristics from medial fractures. Therefore, Pires et al^[17] proposed a new classification system especially developed for coronal plane fractures of the medial condyle, based on the size of the fractured fragment and the presence of comminution (Fig. 3). This new classification system guides the approach choice and fixation strategy. Busch-Hoffa fractures are still categorized as 33-B3.2 (Unicondylar) or 33-B3.3 (Bicondylar) according to the AO/OTA Classification system.^[19–21]

Chandrabose et al^[22] proposed a classification system based on the analysis of computed tomography of 103 patients, stratifying the fracture patterns and the most indicated methods of fixation. Bagaria et al^[23] proposed a radiological classification system for Hoffa fractures based on fracture configuration and consequent optimal treatment strategy along with a review of the literature. However, the authors highlighted that this classification system may be considered as arbitrary as it is not based on any biomechanical or long-term follow-up prognosis analysis.

Busch-Hoffa fractures usually occur as a result of high-energy trauma,^{[11,14,19,24,25]} mostly caused by traffic accidents with the knee in a flexed position. According to Wagih et al,^[26] in the flexed position, the posterior portion of the lateral condyle is the one that receives the first impact. They are almost 10 times less frequent than fractures that affect the proximal part of the femur.^[21] Onay et al^[19] studied the long-term outcomes of treating Hoffa fractures. In a total of 524 distal femur fractures, 14 patients presented a Hoffa fracture, representing 2.67% of total, 12 of which were due to accidents involving bicycles, motorcycles, and other vehicles. Gao et al^[27] observed that, out of a total of 13,702 patients with fractures that occurred within an 8-year interval, 728 (5.3%) presented distal femur fractures, 82 of which were Hoffa patterns. Bel et al^[25] analyzed 163 unicondylar fractures, 18% with a coronal plane pattern. Arastu et al^[28] reported that fractures occur 3 times more frequently on the lateral condyle than on the medial one. The usual injury mechanism was a combination of vertical shear and torsion forces with the knee in a flexed position.^[29,30]

Typical riding posture involves sitting with the knee flexed in a slightly abducted position and this predisposes the posterior aspect of the lateral femoral condyle to receive a traumatic axial load.^[29] In those situations, the lateral femoral condyle is also more subject to direct oblique or lateral impact of the patella, thus causing the Busch-Hoffa fracture.^[16,31] Arastu et al also reported that a possible reason for the fracture could be the application of a force in the vertical plane on the posterior femoral condyle, corresponding to various degrees of knee flexion.^[6,28] Manfredini et al^[29] reported that the trochlear-condylar groove could be a potential structurally unstable point where the fracture could originate and spread to other planes.^[6] Harna et al^[32] reported that the suggested mechanism involves an axial load to the lateral femoral condyle, with the knee in 90° or more of flexion, which produces posterior tangential fracture patterns. Holmes et al^[33] postulated that shear forces act along the fracture line at the femoral condyle, which makes its treatment even more challenging.^[29,34]

Neglected or chronically untreated Busch-Hoffa fractures can evolve to nonunion or malunion, causing pain, disability, and knee arthritis.^[35] The fracture diagnosis can be challenging and often requires adequate clinical analysis, complemented with imaging work-up. White et al^[31] advocate that Hoffa fractures are not easy to visualize on radiographs in anteroposterior views, especially in non-displaced or minimally displaced patterns. Computed tomography is indicated to investigate coronal fracture lines, as well as to better understand the fracture orientation, fragment size, and presence or absence of comminution or joint depression.^{[6,24,25,27,36–38]}

The variation between different aspects of this fracture makes it difficult to delineate a specific treatment protocol.^[6] Nonoperative treatment usually leads to nonunion, malunion and/or joint stiffness due to prolonged immobilization. Therefore, surgical intervention is usually indicated for Busch-Hoffa fractures.^[39,40] Open reduction with internal fixation is the current standard of care.^{[6,15,16,25,37,41]} This rational approach that combines anatomic reduction, joint surface restoration, stable fixation, and early joint mobilization is critical for achieving a satisfactory outcome.^[8,9,42]

Traditionally, coronal plane fractures of the distal femur are treated with screws alone (depending on the fragment size) or associated with posterior buttressing plates.^[7,15,32] Usually, screw directions are perpendicular to the fracture line in the coronal plane and along the longest axis of the femoral condyle. The screws used to fix Busch-Hoffa fractures vary from mini-fragment implants (2.0, 2.4, or 2.7 mm), cortical screws (3.5 or 4.5 mm) or cancellous cannulated screws (3.5, 4.0, 4.5, or 6.5 mm), depending on the size of the fractured fragment and surgeon’s preference. Maheshwari et al^[43] conducted a study with 30 patients to compare the use of conventional and headless screws. The authors reported more complications and failures when using headless screws.

The use of plates positioned with a buttressing role promotes more stability and stiffness, preventing vertical displacement^[4,32] However, depending on the fractured fragment size, placement of buttressing plates may be impossible due to the absence of a posterior metaphyseal zone for plate accommodation. Min et al performed open reduction and internal fixation in 8 patients, using plate and 6.5 mm headless compression screws in P-A direction, considering this treatment strategy an effective alternative for Busch-Hoffa fractures.^[44] However, what is observed in the literature and in clinical practice is that fixations are more commonly performed from the A-P direction, due to the surgeons’ familiarity with the anterior approaches and lower risk of iatrogenic neurovascular injuries. Nevertheless, screws applied in the P-A direction are biomechanically more stable and provide greater fixation stiffness^[31,45–47] Gavaskar et al^[9] reported the outcomes of 18 patients with Busch-Hoffa fractures who were fixed using both, screws in the anterior-to-posterior and posterior-to-anterior directions, not observing differences in reduction and fixation failure. Despite the aforementioned techniques, as reported by Arastu et al,^[28] the ideal stiffness required for fixation of this fracture pattern is still unknown. Harna et al^[32] advocate that internal fixation with low compression force can result in high shear stress at the fracture site, interrupting osteogenesis and promoting nonunion. Singh et al,^[40] observing the treatment of 6 patients where 3 had fixation failures, highlighted the difficulty of reconstructive surgery for this type of fracture. The study of Trikha et al^[8] compiles a review of 32 patients, all treated with open reduction and internal fixation, using lateral approach to address lateral and a medial approach to address medial Busch-Hoffa fractures.

Using P-A fixation, Jarit et al^[48] observed that there is a greater risk of injury to neurovascular structures, especially the peroneal nerve, which is at risk along the medial border of the biceps femoris. In addition, the lateral superior geniculate artery is also at risk. Yao et al^[49] highlighted the difficulty to insert a P-A screw using direct lateral approach, which is more familiar for surgeons and with a lower risk of iatrogenic neurovascular injury. Bel et al^[25] observed that surgical treatment through an adequate anterior approach allows anatomical reduction and stable fixation. However, for screw placement, care must be taken to countersink the screw head to prevent friction between the metal and the cartilage during knee mobilization.^[48,50]

Lu et al,^[39] in a case series of 45 patients with Busch-Hoffa fractures (15 type I, 12 type II and 18 type III), observed that the mean duration of surgery and mean blood loss were significantly higher in the group of patients treated with screws associated with the posterior plate compared to patients treated with screws alone. The likely explanation for this is the need of a more extensive approach, which potentially increases bleeding. Patients treated with screws associated with the plate presented greater range of motion. At the final follow-up, all patients presented fracture healing and progressed without loss of reduction, nonunion, or malunion. Orapiriyakul et al,^[4] observing the size of the fragment, reported the lack of consensus on the approach for fixation of type II Hoffa fractures, since smaller fragments present greater difficulty to be fixed and compromised blood supply can lead to nonunion or osteonecrosis.^[4,15] Gao et al^[27] described important points that should be highlighted, among them the protection of the nutrient artery and the insertion of the collateral ligament, especially in type II Hoffa fractures.

Orapiriyakul et al^[4] analyzed 20 cadaveric specimens and observed that for fragments smaller than 18.3% of the anteroposterior diameter of the medial condyle and 10.1% of the lateral one, the fracture may be inaccessible by the anterior parapatellar approach, being recommended for fractures larger than 28.7% from the medial condyle and 19.9% from the lateral condyle. In fractures smaller than 28.7% for the medial condyle and 19.9% for the lateral condyle, the direct medial extensile approach or the posterolateral approach can be adopted, respectively. The authors also reported that combined approaches can be performed in complex fracture patterns.

The screw diameter, its length, trajectory, as well as the number of threads are directly related to the stability of the assembly.^[4,19] Therefore, there is a need to carry out experimental and numerical studies comparing different implants, in different screw trajectories, with the objective of optimizing the mechanical response, but without losing focus on preserving soft tissues.

The standard fixation method for Busch-Hoffa fractures is the use of screws, with the principle of interfragmentary compression to achieve absolute stability.^[24] In larger fragments, in which there is a posterior metaphyseal fragment, a buttressing plate associated with lag screws improves stability. Although several types of construct have been described, depending on the size of the fragment and presence of comminution, the literature remains controversial with respect to the ideal fixation.

Although closer to a real situation, human cadaveric bones have mechanical properties influenced by age and bone density that vary from sample to sample, which presents itself as a potential bias in a comparative analysis. Variations in anatomy, in the size of anatomical structures and in bone density can contribute to high standard deviations, in addition to the greater difficulty in obtaining cadaveric bones that have the same age profile as individuals susceptible to fractures.^[51,52] Therefore, to carry out experimental tests, some authors choose for the use of synthetic models, since they maintain a standard of mechanical properties and anatomical homogeneity, which allows a more accurate observation of the fixation techniques and thus guarantees methodological consistency.^[29,45] Attention should be paid to the use of synthetic models with mechanical characteristics similar to the bone density of the study population for a given fracture pattern. In the case of Busch-Hoffa fractures, most patients are young adults, which requires synthetic models that simulate the mechanical properties of this population profile.^[53] However, the use of composite femurs can limit the direct comparison of experimental results to a clinical situation, since the absence of soft tissues is still a potential bias.^[54]

Sun et al^[45] experimentally tested 16 adult synthetic femurs to fix Letenneur type I fractures. The mechanical tests were performed evaluating the fixation with 6.5 mm partially threaded screws, 3.5 mm screws and locking compression plate, obtaining greater axial stiffness and load until failure in the group of plate fixation. The authors also reported that, for the treatment of Letenneur type I fractures, fixation with the plate, either in the posterior or lateral position, provides significantly higher stability than isolated screws, regardless the screws trajectory. The authors also emphasized that the lateral plate presented a better behavior than the posterior one, reinforcing that the plate in a posterior position is more difficult to place and shape. However, lateral plate fixation involved 2 fragment fixation elements, while posterior fixation involved only one, which can be understood as a potential bias.^[45]

Yao et al^[49] mechanically studied Letenneur type I fractures, dividing 16 synthetic bones into 2 groups, to observe the effects of varying the trajectories of the 6.5 mm cannulated screws. The authors reported better mechanical performance in crossed screws than in those with a traditional parallel trajectory. Hak et al^[55] experimentally evaluated 20 composite femurs with different fixations among 1 and 2 3.5 mm screws and 1 and 2 partially threaded (cannulated) 6.5 mm screws, finding greater stability in the latter type of fixation for Letenneur type II fractures. Jarit et al^[48] rehearsed 8 pairs of embalmed femurs, creating Busch-Hoffa fractures for simulation, and evaluated 2 types of fixation with 6.5 mm cancellous screws. The authors concluded that the P-A direction provides greater fixation stability than fixation in the A-P trajectory.^[48]

The use of several screws for fixation in biomechanical experiments, as well as the use of screws with a larger diameter, despite providing greater mechanical efficiency, produce greater damage to the joint surface.^[51,55] Yao et al^[49] observed, based on the load-displacement curve, that all the studied specimens exhibited changes in the slope of the curve before catastrophic failure, which indicated loss of fixation, followed by plastic deformation until failure. For biomechanical testing, the specimens were placed on specific bases that vary according to the model of the machine used and the load was applied at a speed that varies from 1 mm/min^[45] to 2 0 mm/min^[55] and a load up to 2000 N.^[45] Figure 4 illustrates an experimental test model for a better understanding of a biomechanical scenario for evaluation of Busch-Hoffa fixation constructs.

Simulation studies are useful for predicting and demonstrating the influence of specific factors on a given system. Since clinical studies can be influenced by several controlled and uncontrolled variables, finite element (FE) models can effectively focus on a single variable, disregarding the effect of other variables and thus, determine the best response for the variable under analysis.^[49] In addition, experimental tests can validate and compare the response of these numerical models, thus performing a smaller quantity of experimental tests and looking at more data using numerical analysis.^[56,57]

Freitas et al^[47] numerically analyzed, from a biomechanical point of view, 4 fixation constructs (4.5 mm cortical screws and 7 mm cannulated screws in the anteroposterior and posterior-anterior directions) for the treatment of Busch-Hoffa Letenneur type II fractures. The authors concluded that fixation with a 7 mm cannulated screw in the P-A direction presented the best mechanical results, causing a decrease in vertical displacement and greater stability. However, evaluating the Von Misses tension, the anteroposterior direction with a 7 mm screw (7 A-P) provided the greatest reduction in tension peak when compared to the different models, thus considering the best effect. The study by Freitas et al,^[47] to the best of our knowledge, is the only one to analyze Hoffa fractures using the FE method, numerically simulating this type of fracture and its surgical solutions.

A summary of clinical studies about the method of fixation, trajectory of fixation and approach is presented in Table 1 for lateral condyle fractures, Table 2 for medial condyle fractures, and Table 3 for bicondylar fractures.