Around 223,000 people in Germany are hospitalized each year due to knee and lower leg injuries, including numerous injuries to the posterior cruciate ligament and the collateral ligaments. Complex injuries of the posterior cruciate ligament and the collateral ligaments are among the most challenging conditions in knee surgery. They require precise diagnostics, a deep understanding of biomechanics, and surgical expertise that only a few centers can offer in this form. The editorial team of the Leading Medicine Guide spoke with one of Germany’s leading specialists, Professor Dr. med. Karl-Heinz Frosch.
„Complex knee injuries usually occur when not just a single ligament is affected, but multiple structures are damaged simultaneously—for example, an anterior cruciate ligament together with the medial collateral ligament, or a posterior cruciate ligament combined with the posterolateral corner or the lateral ligaments. Such injuries can often already be identified through a thorough patient history and careful clinical examination, because the direction of instability provides a strong indication of which ligament is torn. MRI is particularly helpful in the acute phase, for example in posterior cruciate ligament injuries, and offers a high level of diagnostic certainty. However, in cases of long-standing instability, its accuracy decreases significantly, which is why clinical examination plays the most important role“, explains Prof. Dr. Frosch initially about knee injuries and highlights a special feature of the posterior cruciate ligament:
„The posterior cruciate ligament differs from the anterior in many ways. It rarely tears due to a typical twisting injury but is usually the result of high-energy trauma—for example, when the lower leg is forced backward in a motorcycle accident due to impact, or when someone falls directly onto the lower leg with the knee bent. Hyperextension injuries, in which the knee is forced backward, are less common but are almost always associated with additional ligament injuries. Overall, these are injuries typically associated with significant force. Because injuries of the posterior cruciate ligament and the posterolateral corner are rare, there are only a few centers in Europe that focus on them clinically and scientifically. Over time, a particular expertise develops there, and many patients are specifically referred to these centers, as complex knee injuries require specialized experience“.
The posterior cruciate ligament stabilizes the knee by preventing the lower leg from sliding backward relative to the thigh. It plays a central role in stability, particularly at 90 degrees of flexion, working closely with the popliteus complex, which absorbs a large portion of rotational and posterior translation forces. Together, these structures ensure controlled joint motion under load, prevent abnormal movements, and protect the articular cartilage in the long term.
Complex injuries of the posterior cruciate ligament can affect anyone—from children to older adults. In clinical practice, however, a clear pattern emerges: highly active individuals between the ages of 20 and 30 are particularly often affected. From around the age of 50, the incidence decreases significantly, and posterior cruciate ligament injuries are very rare in older patients.
Prof. Dr. Frosch comments: „When such an injury occurs, it typically presents as a combination of instability and pain. Interestingly, injuries to the posterior cruciate ligament are usually significantly more painful than those to the anterior cruciate ligament, although the exact reason is unclear—jokingly, it is sometimes even referred to as ‘PCL – P for Pain.’ It is also important to note that a posterior cruciate ligament injury does not automatically require surgery. An isolated rupture can often be treated conservatively. The problem, however, is that most posterior cruciate ligament injuries are accompanied by additional injuries—particularly to the posterolateral corner. And these additional instabilities are often overlooked. A large UK study shows that such associated injuries are identified on average only 30 months after the first specialist consultation. This explains why posterior cruciate ligament injuries are considered particularly challenging and why specialized centers play an important role“, and adds:
„If a posterior cruciate ligament injury is truly isolated, it can be treated conservatively with good outcomes. The difficulty is that even experienced clinicians often overlook accompanying injuries of the posterolateral corner. And if these are missed, surgical outcomes are generally poor. Many posterior cruciate ligament reconstructions fail not because the ligament itself was poorly operated on, but because additional instabilities were simply not recognized. If it is indeed an isolated injury—which is rare—it can be treated successfully without surgery. However, conservative treatment is considerably more demanding than for the anterior cruciate ligament, because a special brace is required that actively guides the lower leg forward and prevents healing in a fixed posterior sag position. This is precisely what often happens when conservative treatment is inadequate. It is estimated that around 30 percent of conservative treatments fail, leaving the knee in this posterior sag position, which leads to pain and functional problems. Therefore, high-quality conservative therapy that is consistently implemented is essential. Such therapy primarily involves early restoration of mobility and consistent use of the specialized brace for six weeks. This is generally manageable for most patients. Surgery is usually required when additional structures besides the posterior cruciate ligament are injured, most commonly the popliteus complex or the lateral collateral ligament. These combined injuries typically do not respond to conservative treatment. In such cases, surgical intervention is necessary. And yes, it is possible to treat even these complex structures minimally invasively. We are among the few centers worldwide that reconstruct the posterolateral corner arthroscopically. This is technically very demanding, but we have been performing it successfully since 2014. Functionally, the outcomes are just as good as with open surgery, but with smaller incisions, a lower risk of infection, and faster recovery“.
Whether a complex knee injury can be stabilized using conservative methods or requires reconstruction of multiple ligament structures depends on a range of finely tuned biomechanical factors that determine whether the knee can achieve functional stability despite injury or whether load axes are so disrupted that only surgical reconstruction can restore physiological joint mechanics.
Whether arthroscopic reconstruction is sufficient or open surgery is required depends on a set of clear structural and biomechanical criteria that reveal how complex and extensive the injury truly is. The decisive factor is not only which ligament is torn, but how many structures are affected simultaneously and whether anatomical relationships can be restored minimally invasively at all.
View of the knee with posterior cruciate ligament._Henry Vandyke Carter, Public domain
The central question is how well the three key stability components—anterior/posterior translation, varus/valgus stability, and rotational control—are still preserved in the injured knee. If the posterior cruciate ligament or the collateral ligaments are damaged, the knee can only compensate if the remaining structures, especially the muscles, are sufficiently strong, intact, and well-coordinated. If posterior translation cannot be controlled despite muscular activation, the knee loses its essential braking function, making reconstruction almost inevitable. Another decisive factor is the interaction of ligament structures under load. The posterior cruciate ligament works closely with the posterior oblique ligaments, the posterolateral complex, and the collateral ligaments. When several of these structures are injured simultaneously, a combined instability arises that can usually no longer be compensated for with conservative measures.
„The greatest challenge in surgical procedures lies in the fact that we are working with very sharp instruments in an anatomically highly sensitive area—the popliteal fossa. Important nerves and blood vessels run here, which in anterior cruciate ligament surgery are completely outside the surgical field. Therefore, anterior cruciate ligament surgery is hardly comparable to posterior cruciate ligament surgery, and the latter is significantly less complex than arthroscopic treatment of the posterolateral corner. This region has a complicated anatomy, and arthroscopic work must be extremely precise and meticulous to ensure safety. The fact that only a few surgeons master this is evident from the frequency: statistically, there is one posterior cruciate ligament rupture for every twenty anterior cruciate ligament ruptures. Before attempting posterior cruciate ligament surgery, a surgeon should have performed 100 to 200 anterior cruciate ligament procedures. Accordingly, there are only about a dozen surgeons who routinely perform these procedures. For patients, this means they should specifically seek out a specialized center. While high-quality care for anterior cruciate ligament injuries is widely available in Germany, this is only the case for posterior cruciate ligament injuries in selected centers. Combined ligament reconstructions are so important because the posterior cruciate ligament almost never tears in isolation. Usually, the lateral collateral ligament and parts of the popliteus complex are also affected. The popliteus stabilizes the knee at 90 degrees of flexion against external rotation and absorbs about half of the force generated during posterior translation. If the posterior cruciate ligament is reconstructed but the popliteus complex is overlooked—which unfortunately happens frequently—the new ligament becomes overloaded and loosens quickly. This is why combined procedures are necessary, in which the posterior cruciate ligament is stabilized or reconstructed together with the popliteus complex and often the lateral collateral ligament“, emphasizes Prof. Dr. Frosch.
Early recognition of whether a posterior cruciate ligament injury also involves the posterior oblique ligaments or collateral ligaments is only possible if the typical biomechanical patterns of these combined injuries are consistently taken into account. In such cases, the knee shows not only pure posterior instability but also loses its ability to control rotation and varus/valgus forces. These changes provide the decisive diagnostic clues.
Instabilities of the knee joint must therefore be assessed in a highly differentiated manner. It is crucial to identify all components of instability through clinical examination, which requires significant experience. All of these instability components should then also be addressed surgically, which can be technically very demanding. In cases of combined injuries, isolated reconstruction of individual structures is generally not sufficient to restore physiological joint mechanics.
„Whether arthroscopic reconstruction is sufficient or open surgery is required depends on a set of clear structural and biomechanical criteria that reveal how complex and extensive the injury truly is. The decisive factor is not only which ligament is torn, but how many structures are affected simultaneously and whether anatomical relationships can be restored minimally invasively at all. The benefit of combined reconstruction is particularly evident in posterior cruciate ligament injuries, which almost never occur in isolation. If the posterolateral complex, posterior oblique ligaments, or collateral ligaments are also involved, a complex rotational and lateral instability develops that cannot be compensated for conservatively or with isolated reconstruction. Patients who receive reconstruction of only a single ligament in such situations often develop persistent instability, an insecure gait pattern, and progressive overload of the medial or lateral joint compartment. Even in high-energy trauma—such as traffic accidents or severe sports injuries—patients benefit from combined reconstruction, as multiple ligament structures usually fail simultaneously. If only part of the injury is addressed, joint mechanics remain impaired, leading to secondary meniscal damage, cartilage wear, and ultimately early osteoarthritis. Another important group includes patients with pronounced rotational instability. If the knee gives way during rotational movements, ‘opens outward,’ or shows a pronounced ‘giving-way’ sensation, more than one ligament is almost always involved. Combined reconstruction restores the coordinated function of ligament complexes and prevents uncontrolled rotation of the tibia under load—a central mechanism in cartilage damage“, emphasizes Prof. Dr. Frosch.
Age, activity level, and associated injuries act as three interconnected variables that determine whether a conservative approach can still provide sufficient stability or whether surgical reconstruction is necessary to maintain long-term knee function. Each of these variables alters the biomechanical demands placed on the joint—and thus the likelihood of success of each treatment approach.
A partial knee replacement following a tibial plateau fracture—such as in the case of elite ski racer Lindsey Vonn—and then returning to world-class skiing is truly something that impresses even specialists. Prof. Dr. Frosch comments: „This is only possible through an extraordinary combination of determination, athleticism, and body awareness. A partial knee prosthesis can restore function well, but it does not replace a natural joint. The menisci are missing, proprioception in the knee changes, and certain movement feedback that a healthy knee provides is reduced. All the more remarkable that Lindsey Vonn was able to compete at the highest level in the World Cup. This highlights both her personal capabilities and the fact that modern prosthetic technology can achieve a surprisingly high level of function. That her career likely ended after her last fall is not surprising given the stresses of alpine skiing. While a ski boot stabilizes the ankle and provides some overall guidance to the leg, the crucial work in skiing comes from the knee. Especially at high speeds, during rapid directional changes, and under extreme forces, the knee is the central joint. This makes it even more remarkable that she was able to perform at this level with a partial knee prosthesis. For patients who are not competing in the World Cup, her story is nevertheless encouraging. It shows that with the right treatment, a good prosthesis, and above all consistent participation in rehabilitation, a great deal can be achieved. Successful rehabilitation requires restoring strength, mobility, and coordination, while inflammation in the knee must be gradually reduced through physical measures. The real art lies in experienced physiotherapists recognizing exactly which phase the patient is in and what benefits or harms the knee at that moment. Rehabilitation is highly individualized, and no two patients follow the same course. The process does not end after rehabilitation. The patient continues to be monitored by specialists, as the first six months are particularly decisive for long-term knee function. The joint should be regularly assessed within the first year, but the most important course is usually set within the first six months“.
Robotics plays a major role in knee surgery—however, primarily in artificial knee joint replacement rather than in ligament surgery. In endoprosthetics, a robot can compensate for small inaccuracies by the surgeon and significantly improve the precision of implantation.
„The robot does not work autonomously like a car in assisted driving mode but requires a highly experienced surgeon to guide it. Nevertheless, it provides a few additional degrees of precision, and that is exactly what makes it so valuable. We use a system from DePuy, the VELYS robot, which we have been using for several months. It is particularly helpful in post-traumatic knee osteoarthritis, such as after workplace accidents or old tibial plateau fractures. In such cases, the anatomy is often so altered that orientation becomes difficult. The robot can achieve a level of precision that even very experienced surgeons can hardly match, as it works independently of deformed structures and aligns the prosthesis exactly. However, mastering such a system takes considerable time. Training takes months, including courses, model-based training, and cadaver practice before operating on patients. This only works if one is already a very experienced knee surgeon—otherwise, one should not be using a robot. Not every procedure is performed robotically. In cases of simple osteoarthritis without significant axis deviation, a robot is not necessarily required. It becomes particularly useful from about ten degrees of axis deformity and generally in post-traumatic cases, where it is almost always used“, says Prof. Dr. Frosch and concludes our conversation by emphasizing:
„We use robotics at the BG Hospital (Berufsgenossenschaft Hospital), while treatment of posterior cruciate ligament injuries is performed at a high level both there and at UKE. Together, both institutions perform around 1,600 knee surgeries per year. Particularly notable is the number of tibial plateau fractures—here we are among the leading centers in Germany. Nationwide, there are only five hospitals that perform more than 50 posterior cruciate ligament surgeries per year; both UKE and the BG Hospital are among them, which is remarkable given how rare these injuries are. For the future, I would like to see less bureaucracy and more genuine patient-centered care—and a smart hospital policy that strengthens specialization rather than making it more difficult“.
Thank you very much, Professor Dr. Frosch, for this insight into complex knee surgery with a focus on the posterior cruciate ligament!
- Director of Trauma Surgery and Orthopedics at UKE, and one of Germany’s leading knee specialists.
- Expert in complex knee surgery, cruciate ligament and meniscus injuries, osteoarthritis treatment, patellar dislocations, and severe injuries of the lower extremity.
- Focuses on joint-preserving therapies and avoids artificial knee replacements whenever possible—particularly relevant for professional and elite athletes.
- Extensive expertise in arthroscopic knee surgery, corrective osteotomies, meniscus replacement, cartilage cell transplantation, and osteochondral transplantation (OCT).
- Performs approximately 70% revision surgeries, particularly involving the posterior cruciate ligament.
- One of the few specialists who perform posterior oblique ligament surgery entirely arthroscopically.
- Many years of experience in treating congenital and post-traumatic deformities.
- Former competitive athlete with close ties to the Olympic Training Center Hamburg/Schleswig-Holstein.
