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Letters to the Editor

Hannover, Germany

Dear Editor:

We would like to comment on the article "Medial Opening Wedge Tibial Osteotomy and the Sagittal Plane" by Rodner et al (September 2006, page 1431–1441). As stated in the article, it is well known that a change of the sagittal plane of the tibial plateau considerably influences the kinematics of the knee joint. This was first mentioned by Dejour and Bonnin² in an article that used lateral radiographs in patients with monopodal stance and later by others.³ However, it is completely untrue that the study by Rodner et al is the first to show the influence of the tibial slope on the tibiofemoral cartilage pressure. Our group published an experimental study on exactly the same subject 2 years ago.¹ It is a great pity that this article, which is in fact the only study ever published in a peer-reviewed journal on this topic, was totally ignored by Rodner et al.

Furthermore, drawing on our experimental and clinical experience regarding knee joint-related osteotomies, tibiofemoral cartilage pressure measurements, and posterior tibial slope, we would like to make some critical remarks on the experimental setup of the study by Rodner et al, which seems to have some weaknesses.

1. An Arthrex spacer (Puddu) plate was mounted in 2 different positions at the medial proximal tibia, leading to a valgus with or without an increase of the tibial slope. It is obvious that the tibiofemoral contact pressure in both the medial and the lateral compartment was thus influenced not only by the sagittal but also by the frontal change of the orientation of the tibial plateau. This 2-plane change of the loading conditions represents a serious bias in the experimental conditions. The valgus, which was simultaneously created by the applied osteotomy technique, largely affected not only the quantity but also the topographic distribution of the tibiofemoral contact pressure. Therefore, it is not possible to conclude that is only related to the sagittal anatomy change, the tibial slope. In contrast, in our study, an osteotomy was performed that only influenced the tibial slope of the plateau without a change in the varus-valgus orientation.
   
2. Only 2 different tibial slopes were analyzed, in contrast to our study, in which 5 different slope values in 5° increments were tested. The amount of the slope change in the study by Rodner et al was rather imprecise: the slope was not directly and reproducibly altered but was indirectly changed by simply applying a spacer plate at the proximal medial tibia in 2 different positions. These plate positions were selected by the surgeon according to some anatomical landmarks, which naturally can vary between various individual specimens. Furthermore, for osteotomy and plate positioning, the medial collateral ligament was reflected back from the tibia, which means that it was released from its distal insertion. It is well known that any collateral ligament detachment has a huge impact on the tibiofemoral intra-articular pressure. Thus, another bias was created by this ligament release.
   
3. The loading model of the knee has several weaknesses and seems not very physiologic: it was carried out using a semicon-strained fixture with rigid fixation of the knee flexion angle with only 2 flexion angles tested. No dynamic movement of the joints between flexion and full extension was performed, but a very simple axial loading model along the tibial axis was applied. No muscular forces of the quadriceps or the hamstrings were simulated. Moreover, the loading force was 500 N at its maximum, which is severely less than physiologic in situ forces of the knee, which have been shown to exceed 3000 N.
   
4. The Tekscan films were inserted underneath the medial and lateral menisci. This means that the load from the femur onto the films was indirectly transduced to the Tekscan measuring system through the substance of the menisci, which cover the medial and lateral tibial plateau in total by about 40% to 50%. So a shift of the contact pressure into the posterior area of the tibiofemoral joint compartment was buffered by the posterior part of the meniscus. Thus, the posterior pressure values, which were measured through the filter of the meniscus, cannot directly be compared with the pressure values, which were measured more anteriorly, where there is potentially no meniscus between femoral condyle and pressure film.
   
5. As correctly stated by Rodner et al, another severe weakness is no experiments without osteotomy with the knees in native condition were performed. This lack of a control group ("normal knees") makes any conclusion impossible that compares a slope alteration with a normal knee without surgery.
   

In summary, we feel that because of the mentioned weaknesses in the experimental setup, the impact of results of the study by Rodner et al is very limited for clinical use. The cited literature is incomplete because the only peer-reviewed published experimental study on the same subject was not included.

REFERENCES

1. Agneskirchner JD, Hurschler C, Stukenborg-Colsman C, Imhoff AB, Lobenhoffer P. Effect of high tibial flexion osteotomy on cartilage pressure and joint kinematics: a biomechanical study in human cadaveric knees. Winner of the AGA-DonJoy Award 2004. Arch Orthop Trauma Surg. 2004;124:575–584.
2. Dejour H, Bonnin M. Tibial translation after anterior cruciate ligament rupture. Two radiological tests compared. J Bone Joint Surg Br. 1994;76:745–749.
3. Giffin JR, Vogrin TM, Zantop T, Woo SL, Harner CD. Effects of increasing tibial slope on the biomechanics of the knee. Am J Sports Med. 2004;32:376–382.[Abstract/Free Full Text]

Authors’ Response

Farmington, Connecticut

Our study demonstrates that changes in sagittal plane alignment occur during opening wedge tibial osteotomy, depending on how the opening is performed and maintained. The consequence is a measurable difference in articular contact pressure distribution, and in the patient with an ACL-deficient knee (many of whom have meniscal deficiencies), the resulting posterior shift of joint contact pressure could have an undesirable effect.

We regret not citing the work of Agneskirchner et al from their November 2004 article published in Archives of Orthopaedic Trauma Surgery.¹ The omission was unintentional, and we apologize for missing the publication while revising our own. We made no claim of being first to study this subject. The 2 articles adopted different osteotomy models and different knee loading models, each with their own limitations. Our article addressed many of the experimental limitations brought forth by Agneskirchner and Lobenhoffer, which we clarify further below.

1. The authors criticize that we introduced both a valgus change and a change in sagittal slope. Indeed, our intent was to simulate what surgeons do clinically, especially in the ACL-deficient condition. The study by Agneskirchner et al¹ changed only the tibial slope to isolate that effect. In actual practice, the most common reason for performing proximal tibial osteotomy is to correct varus malalignment. The manner in which we performed the surgery is very similar to what is done clinically, and we reason that evaluating change only in tibial slope has little clinical application. We believe it is far more important for surgeons to be aware of what happens to the sagittal alignment when performing an opening wedge osteotomy.
   
2. The authors criticize the "indirect" changes in tibial slope. This is a matter of clinical perspective. Agneskirchner et al¹ imposed known incremental sagittal slope changes regardless of anatomy. We chose to apply a known opening wedge spacer/plate and evaluate it in 2 different anatomical locations that surgeons use when performing this procedure. Our perspective was to perform an opening wedge as performed clinically, measure the resulting changes in tibial slope, and evaluate the corresponding difference in pressure distribution. The authors also criticize the "release" of the medial collateral ligament. Again, elevation or release of this structure is clinically relevant. If the surgeon desires to place the plate or open the wedge posteriorly, some violation of the medial collateral ligament (MCL) must occur. We chose to elevate it in a consistent manner, and because it was performed in each specimen in a consistent sequence, we believe that the effect of MCL release was uniform and did not bias the conclusions of the study.
   
3. Our article and that of Agneskirchner et al¹ used very different knee joint loading strategies. Agneskirchner et al simulated a dynamic, quadriceps-activated leg extension motion (an open-kinetic chain). Because the model is dynamic through a flexion arc, quadriceps activation swings the limb freely toward extension without incurring additional joint contact force associated with weight bearing and without hamstring activation to mediate anterior drawer.^2,3 The contact forces increase near joint extension as the anterior contact of the tibia creates a fulcrum to lock against hyperextension. Joint pressures were reported at this position of full extension.
   
4. Rodner et al⁵ simulated a weight-bearing model (closed kinetic chain) with unconstrained knee joint motion except for knee flexion, which was fixed. Importantly, we applied a known compressive joint load, allowing for appropriate post hoc integration and normalization (correction) of contact pressure magnitude data. This is critical when using Tekscan sensors because of the inherent creep and electronic drift associated with the sensor piezoresistive ink.⁴ This variable correction of up to 40% is often unappreciated and overlooked by users of the instrumentation, and it was not possible in the leg-lift model by Agneskirchner et al¹ because the applied joint forces were not known. We chose an input of 500 N to ensure control of the joint and to prevent saturation of individual sensor cells. A similar load magnitude has been used in other studies of knee joint contact pressure, as referenced in our article.⁵
   
5. We are surprised by this criticism and contend that appropriate use of the sensor includes submeniscal placement. The goal of our study was to measure articular pressures on the tibial plateau. Placement above the menisci implies measurement of femoral contact forces along a curved arc, with potentially severe aberrations in pressure recordings.⁴ The Mylar film composing the sensor does not conform to femoral or meniscal geometry, resulting in artifactual sensor excitations associated with sensor wrinkle. The addition of Teflon foil coverings to "protect" the sensor during joint motion would likely moderate the wrinkle artifact by thickening and stiffening the construct, thereby limiting conformity and reducing accuracy of contact area measurements.
   
6. Rodner et al⁵ cited a number of reasons for not first performing an intact control measurement of contact pressure magnitude and distribution. Our intent was to compare differences in tibial slope (in 2 planes) and contact pressure distribution that occur when an opening wedge osteotomy is performed on the medial tibia in an anterior location versus a posterior location. We were additionally interested in this condition as it pertains to the ACL-deficient knee, for which the lack of intact pressure mappings does not detract from our findings.
   

We stand by the statement that the effects of changes in tibial slope on articular contact pressures are "unknown." Mechanical studies cannot reproduce the in vivo situation. Our work and that of Agneskirchner et al¹ provide some insight but collectively fall short of reproducing the relevant loading conditions in the actual patient.

Finally, we strongly believe that despite the limitations in our study and that of Agneskirchner et al,¹ changes in sagittal plane alignment occur during opening wedge tibial osteotomy depending on how the opening is performed and maintained. This has a measurable change in pressure distribution, and in the patient with an ACL-deficient knee (many of whom have meniscal deficiencies), a shift of pressure occurs posteriorly, which could have undesirable effects.

REFERENCES

1. Agneskirchner JD, Hurschler C, Stukenborg-Colsman C, Imhoff AB, Lobenhoffer P. Effect of high tibial flexion osteotomy on cartilage pressure and joint kinematics: a biomechanical study in human cadaveric knees. Winner of the AGA-DonJoy Award 2004. Arch Orthop Trauma Surg. 2004;124:575–584.
2. Li G, Rudy TW, Sakane M, Kanamori A, Ma CB, Woo SL-Y. The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL. J Biomech. 1999;32:395–400.[CrossRef][ISI][Medline][Order article via Infotrieve]
3. More RC, Karras BT, Neiman R, Fritschy D, Woo SL, Daniel DM. Hamstrings—an anterior cruciate ligament protagonist: an in vitro study. Am J Sports Med. 1993;21:231–237.[Abstract/Free Full Text]
4. Otto JK, Brown TD, Callaghan JJ. Static and dynamic response of a multiplexed-array piezoresistive contact sensor. Exp Mech. 1999;39:317–323.[CrossRef]
5. Rodner CM, Adams DJ, Diaz-Doran V, et al. Medial opening wedge tibial osteotomy and the sagittal plane—the effect of increasing tibial slope on tibiofemoral contact pressure. Am J Sports Med. 2006;34:1431–1441.[Abstract/Free Full Text]

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