The Knee
Volume 18, Issue 1 , Pages 51-54, January 2011

Biomechanical analysis of posteromedial tibial plateau split fracture fixation

  • Zhi-Min Zeng

      Affiliations

    • Department of Orthopaedic Surgery, Shanghai Sixth People's Hospital affiliated to Shanghai JiaoTong University. 600 Yisan Road. Shanghai, 200233, PR China
    • Tel.: +86 21 64369181x8800; fax: +86 21 64083239.
    • ,
  • Cong-Feng Luo

      Affiliations

    • Department of Orthopaedic Surgery, Shanghai Sixth People's Hospital affiliated to Shanghai JiaoTong University. 600 Yisan Road. Shanghai, 200233, PR China
    • Corresponding Author InformationCorresponding author. Tel.: +86 21 64369181x8800; fax: +86 21 64083239.
    • ,
  • Sven Putnis

      Affiliations

    • Trauma & Orthopaedic Department, St. George's Hospital, London, UK
    • ,
  • Bing-Fang Zeng

      Affiliations

    • Department of Orthopaedic Surgery, Shanghai Sixth People's Hospital affiliated to Shanghai JiaoTong University. 600 Yisan Road. Shanghai, 200233, PR China
    • Tel.: +86 21 64369181x8800; fax: +86 21 64083239.

Received 26 September 2009; received in revised form 28 December 2009; accepted 6 January 2010. published online 01 February 2010.

Article Outline

Abstract 

The purpose of this study was to compare the biomechanical strength of four different fixation methods for a posteromedial tibial plateau split fracture. Twenty-eight tibial plateau fractures were simulated using right-sided synthetic tibiae models. Each fracture model was randomly instrumented with one of the four following constructs, anteroposterior lag-screws, an anteromedial limited contact dynamic compression plate (LC-DCP), a lateral locking plate, or a posterior T-shaped buttress plate. Vertical subsidence of the posteromedial fragment was measured from 500N to 1500N during biomechanical testing, the maximum load to failure was also determined.

It was found that the posterior T-shaped buttress plate allowed the least subsidence of the posteromedial fragment and produced the highest mean failure load than each of the other three constructs (P=0.00). There was no statistical significant difference between using lag screws or an anteromedial LC-DCP construct for the vertical subsidence at a 1500N load and the load to failure (P>0.05).

This study showed that a posterior-based buttress technique is biomechanically the most stable in-vitro fixation method for posteromedial split tibial plateau fractures, with AP screws and anteromedial-based LC-DCP are not as stable for this type of fracture.

Keywords: Split fractures, Tibial plateau, Posteromedial fragment, Fixation, Biomechanics

 

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1. Introduction 

A posteromedial tibial split fracture is a notable injury pattern of the medial plateau, with the fracture line appearing in the coronal plane with a separate posteromedial osteoarticular fragment of variable size [1]. The occurrence of this “key fragment” has previously been identified, Hohl first described a unicondylar coronal plane split fracture of the medial tibial plateau in 1967 and noted that this injury should be considered as a fracture-dislocation [2]. Since that time, the posteromedial fragment was reported intermittently in the literature. Recently, Barei and Higgins presented the more detailed description of the incidence and morphology of posteromedial fragment in bicondylar tibial plateau fractures [1], [3].

The posteromedial fragment, and its often significant displacement, seems to be under-recognised or recognised and undertreated. Clinically, ignoring this fragment can lead to distal displacement, with posterior subluxation of the medial femoral condyle [1]. Failure of primary fixation results in instability and may also lead to a secondary varus deformity and deterioration in patient functional outcome [4].

The management of this kind of injury remains controversial. Although small stable fragments can be treated by casting with the knee in extension, open reduction and internal fixation is indicated for large displaced fractures to restore joint congruity and stability [5]. There are several approaches and methods reported in the literature for reduction and fixation of a posteromedial tibial plateau shearing fracture [6], [7], [16], [17]. However, to our knowledge, there is no published biomechanical study evaluating the different fixation methods available. The purpose of our study was to compare the biomechanical performance of four different types of fixation in use today: anteroposterior lag-screws, an anteromedial LC-DCP, a lateral locking plate and a posterior T-shaped plate. A synthetic tibia simulating the split fracture model was used. Our hypotheses were that there would be significant differences in the degree of stabilisation provided by these four constructs.

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2. Material and methods 

2.1. Preparation 

Twenty-eight right synthetic tibiae (synbone, type1110, Synbone AG, Swiss) were used to create models of posteromedial tibial plateau split fractures. They were purchased from a single manufacturing batch ensuring the same material properties. With the consistency in their material and geometry, the variability within specimens can be better controlled. A thin blade saw was used to create the osteotomy, simulating a posteromedial split fracture. The fracture model was made based on the fracture morphology described by Higgins: the mean articular fragment angle and sagittal angle of the posteromedial fragment were −21.4 and 73°, respectively, and there was greater than 5mm of articular displacement in 55% of cases [1]. Therefore, in order to control the variability within specimens, 25° of the articular fragment angle and 75° of the sagittal angle were chosen respectively in model designing. The initial cut was made from the posteromedial articular surface directed in a posterior direction, exiting the posterior cortex approximately 4cm distal to the joint line, making the sagittal angle equal to 75° [Fig. 1].

2.2. Groups 

The fracture models were then anatomically reduced under direct vision, temporarily fixed with two K-wires, and instrumented with one of following four constructs: (A) two anteroposteriorly placed 6.5-mm inter-fragmentary lag screws (Kanhui medical limited, Changzhou city, China PR), (B) an anteromedially placed 4.5mm six-hole LC-DCP (Kanhui medical limited, Changzhou city, China PR), (C) a laterally placed five-hole fixed-angle proximal tibial locking plate (LISS, Synthes, Paoli, PA), (D) a posteriorly placed 3.5-mm six-hole T-shaped buttress plate (Kanhui medical limited, Changzhou city, China PR) [Fig. 2]. When a T-shaped plate or LC-DCP were adopted, they were bent to fit the bone contour and fixed with cortical screws. Each specimen was reduced and fixed by a single orthopaedic surgeon. There were seven specimens in each fixation group making it possible to detect a difference of 100N in the mean fixation strength with a statistical power of 80% and alpha error at 0.05 [8].

2.3. Biomechanical testing 

Each specimen was mounted vertically on the table of a material testing machine (Instron 5569, USA). A synthetic femur (synbone, type2200, Synbone AG, Swiss) harvested with a power saw at a length of 5cm (measured vertically from the intercondylar notch) was used as an applicator in order to deliver forces on both tibial plateau surfaces simultaneously, and was clamped on the upper side of the material testing machine. Subsidence was defined as the displacement(less than 3mm) of the posteromedial fragment. During evaluation, relative motion of the posteromedial fragment was measured simultaneously with an electronic gauge (Type FCS1-5-15, Shanghai City, China PR). The gauge length and resolution was 15 and 0.005mm, respectively. Fig. 3 illustrates the testing setup and position of the gauge.

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  • Fig. 3 

    A photograph showing the experiment setup. The subsidence of the osteotomized fragment was measured with an electronic gauge in the vertical axis using an inserted K-wire.

After positioning the instrumented specimen, a vertical compressive force was applied with a loading and unloading speed of 1mm/min. The specimens were first axially loaded to determine the post-fixation stiffness of the construct, and then loaded to failure. Three different load levels using peak forces of 500newton (N), 1000N and 1500N were used. Initial pilot study data found that there were no fixation failures within these limits, with failure load defined as the applied force that produced a 3mm fracture displacement [9].

2.4. Statistical analysis 

Data were normally distributed, indicating the use of parametric statistical methods. Thus a Least-Significant Difference (LSD) post hoc multiple comparison was used for both the subsidence and the load to failure data. Data were analyzed using SPSS 16.0 statistical software (SPSS Inc, Chicago, IL), statistical significance was accepted for P<0.05.

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3. Results 

The subsidence of the posteromedial fragment under three level axial loads is summarised in Fig. 4. The mean subsidence at a 500N load for the four fixation constructs was 0.459, 0.365, 0.264 and 0.128mm, respectively. This was statistically significant (P=0.00). Similarly, mean subsidence at a 1000N load and a 1500N load for the posterior T-shaped buttress plate was significantly smaller than any of the other constructs (P=0.00). There was no statistically significant difference in the vertical subsidence between lag screws and anteromedial LC-DCP at a 1500N load (P=0.098).

  • View full-size image.
  • Fig. 4 

    The subsidence of the posteromedial fragment under three level axial loads. Values are mean displacements. P values were calculated with LSD. There was statistically significant difference between four groups (P=0.00), except for the data of LC-DCP and lag-screws under 1500N(P=0.098).

The load to failure for each construct is shown in Fig. 5. The posterior T-shaped buttress plate was able to absorb more load (3554±263N) prior to failure than the other three constructs (P=0.00). The load to failure of the posteromedial fragment was 2979±139N for the lateral locking plate, 2403±204N for the anteromedial LC-DCP, and 2258±172N for the AP lag screws. The differences between lag screws and anteromedial LC-DCP method was not statistically significant (P=0.18).

  • View full-size image.
  • Fig. 5 

    The average of load to failure sustained by the four constructs. The posterior T-shaped buttress plate was able to absorb more load before failure than the other three constructs (P=0.00).

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4. Discussion 

This study was performed to compare the vertical subsidence and the load to failure in a posteromedial tibial plateau split fracture model stabilized by four different types of fixation in use today. Under the in-vitro conditions of this study, the posterior T-shaped buttress plate had significantly better biomechanical properties than each of the other three constructs.

Posteromedial tibial plateau split fractures are not uncommon, approximately one third of bicondylar tibial plateau fractures have an identifiable coronal plane posteromedial fragment [3]. The mechanism involved in this fracture pattern may be one of knee flexion, varus, and internal rotation of the medial femoral condyle [10]. This type of fracture pattern is more significant than others affecting the tibial plateau because of the instability within the knee joint that it produces. Most authors agreed that malalignment related to poor fixation and associated soft tissue injuries were two important reasons for a poor prognosis [4], [10], [11].

There are key objectives when treating tibial plateau fractures: anatomical reduction of the articular surface, maintenance of normal knee alignment, and provision of sufficient stability to allow early movement [12], [13]. Management of the posteromedial tibial plateau split fracture remains difficult and controversial. Successful treatment requires an understanding of the different fracture patterns. Classification of these types of fracture using the most widely adopted Schatzker [REF] and AO classifications [REF] do not evaluate the fracture effectively, failing to provide enough information to plan treatment [4], [14]. The AP radiograph of this injury often shows little displacement and analysis of the lateral radiograph and a three dimensional understanding of the fracture morphology using CT scanning is therefore essential [15].

Several internal fixation techniques with different clinical results were reported in the literature. Whilst undisplaced posteromedial fragments have be fixed with screws from the anterior incision, Ali et al. demonstrated that when inter-fragmentary screw fixation failure occurs it is always on the medial side [5], [8]. The most commonly used approach to treat medial tibial plateau fractures is fixation with an anteromedial plate through a medial parapatellar incision [16]. This approach however does not give adequate exposure. Attempts to reduce and fix the posteromedial “key fragment” may be difficult and result in extensive dissection of the soft tissue, whilst incomplete restoration of the joint surface results in chronic postero-inferior joint subluxation, osteoarthritis and pain.

The posteromedial fragment often appears to be part of a bicondylar tibial plateau fracture which is associated with severe soft tissue damage. The increased exposure of the soft tissues and bone with dual incisions is potentially problematic. Some authors have suggested that with the advent of locking plates, stable fixation of bicondylar fracture can be achieved solely from the lateral side [17], [18]. However, whether a lateral fixed-angle plate can provide sufficient stability to the posteromedial fragment is controversial. Gosling et al. reported using a single lateral locking plating for the treatment of bicondylar tibial plateau fractures, 14% patients showed substantial loss of reduction [19]. Ratcliff et al., in a biomechanical study, used a cadaveric medial tibial plateau fracture model to compare lateral locked plating with medial buttress plating, they found that a medial buttress plate provided significantly greater stability in static loading [20], similar to our result. The medial portion of the bicondylar tibial plateau fracture may be the weakest link in the lateral locking plate system. It is preferable to augment the lateral locking plate with a medial or posteromedial buttress plate, to avoid varus collapse [20].

The posteromedial key fragment may displace distally and medially especially when the knee is flexed, a posterior-based buttress plate fixation was therefore proposed to maintain stability [6], [21]. Bruuner et al. described a posterior approach for the direct reduction and fixation of the posteromedial fragment, using a buttress plate placed to stabilize the fragment, after a mean follow-up period of 39months all patient were highly satisfied with the postoperative result [21]. Weil et al. reported a posteromedial supine approach for a medial plateau fracture, using a T-shaped plate for fixation, the articular malreduction rate was only 4%. This approach leaves an adequate skin bridge (usually greater than 7–8cm) if an anterolateral incision is additionally required for lateral plateau fixation [22].

The data from our study also indicated that a posterior T-shaped buttress plate produced significantly greater stability in controlling the subsidence of the posteromedial fragment under axial loading. A lateral locking plate allowed greater subsidence of the fracture fragment than a buttress plate in this unstable injury. In order to understand these results, it is helpful to consider the principles of the different fixation methods used in our study. Inter-fragmentary screws compress the fracture and produce friction to resist the shear displacement of a fragment, the loads can only be transferred at the screw–fragment interface [23]. When fixed with a lateral locking plate, the lateral screws are positioned in the coronal plane and are frequently parallel to the fracture line, which may fail to capture the posteromedial fragment [22]. Locking screws directed across from the lateral plate to secure the medial fragment possess limited ability to lag the medial fragment and rely on the lateral screw–plate interface to withstand shearing forces [20]. A posterior buttress plate can be conformed to lie closely to the tibial profile and anchors to strong medial cortical bone, providing strong support to the posteromedial fragment. There is also provision of additional anchorage, apart from that in the tibial metaphysis, by supporting the fragment on a beam resulting in a relatively even load transfer at the screw–fragment interface [23].

In our research, synbone models were used for biomechanical test. The obvious advantages of using synbone are the assurance of uniformity, consistency in its material and geometry, and therefore its mechanical properties [24]. With such a model, the performance of fixation devices can be evaluated more reliably since the variability within specimens can be better controlled.

Several limitations of this study must be considered. Soft tissue tension or attachments may contribute to the alignment and maintenance of fracture fragments in a reduced position [25], our research did not include the soft tissue factors and therefore may differ from the clinical situation. Secondly, our biomechanical study had optimal reduction of the key fragment, which may be a challenge for every clinical case. Furthermore, osteopenia and fracture comminution may affect the stability provided by traditional plate (relying on screw–bone interface), and locking plate (relying on the screw–plate interface) appeared less sensitive to bone quality. Despite these limitations, we feel that our methodology is still valid for the purpose of this study.

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5. Conclusion 

The results of this study demonstrate that a posterior-based buttress technique is biomechanically the most stable in-vitro fixation method for the posteromedial split tibial plateau fracture, whilst AP screws and anteromedial-based LC-DCP are not indicated. When a lateral locking plate is planned, it is preferable to augment the fixed-angle construct with a posteromedial buttress plate to stabilize the key fragment.

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Acknowledgment 

We thank Shang-Chun Guo at the Shanghai Sixth People's Hospital affiliated to Shanghai JiaoTong University, for her assistance and support during this biomechanical study.

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References 

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PII: S0968-0160(10)00007-4

doi:10.1016/j.knee.2010.01.006

The Knee
Volume 18, Issue 1 , Pages 51-54, January 2011

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