Surgical Treatment of Iatrogenic Patella Baja

Abstract

Purpose of Review

Patella baja is characterized by a loss of patellar height and can develop as either an acute or chronic complication following a knee injury or surgical procedure. The purpose of this review is to describe the diagnosis and management of patella baja and highlight the senior author’s surgical technique.

Recent Findings

The pathogenesis of patella baja involves a complex interaction between quadriceps dysfunction, immobilization, and inflammation leading to infrapatellar scarring and adhesions. It is associated with fractures about the knee and can result as a complication of surgical procedures such as anterior cruciate ligament (ACL) reconstruction, particularly bone-patellar tedon-bone autografts, high tibial osteotomies (HTOs), tibial tubercle osteotomies (TTOs), and total knee arthroplasties (TKAs). Patients with patella baja can have limited knee range of motion, anterior knee pain, significant weakness with active knee extension, and an extensor lag. Surgical intervention is indicated in cases of symptomatic patella baja. Treatment strategies include tibial tubercle proximalization, patellar tendon lengthening, and patellar tendon reconstruction. Allografts and autografts can be utilized to augment tendon lengthening or reconstructive procedures. Various small case series have reported favorable outcomes for these procedures.

Summary

The treatment of patella baja is challenging and little consensus exists on optimal management, as much of the literature is limited to small case series. The preferred surgical technique of the senior author involves an end-to-end patellar tendon lengthening with hamstring autograft augmentation.

Introduction

Patella baja, also referred to as patella infera, is a condition involving decreased patellar height. It is characterized by three features: a shortened patella tendon, a decreased distance between the inferior pole of the patella and the proximal articular surface of the tibia, and a distal position of the patella in the femoral trochlea [1]. Patella baja can be congenital or can be acquired as the result of a traumatic injury, a femoral nerve injury, or a surgical procedure [2].

The extensor mechanism is composed of the quadriceps musculature and tendon, patella, patellar tendon, and the retinaculum of the knee. The patella lies deeply embedded in the quadriceps and patellar tendon, linking these two tendons, and displacing the extensor mechanism anteriorly. This acts to increase the mechanical advantage of the quadriceps [3]. Alterations in patellar height disrupt this biomechanical relationship, resulting in abnormal joint reactive forces. In patella baja, rather than disengaging with the trochlea during terminal extension, the patella engages in the trochlea throughout all ranges of motion, leading to anterior knee pain and impingement. Moreover, an extensor lag and reduced range of motion can occur as the result of a decreased lever arm [2, 3].

The treatment of patella baja is challenging and little consensus exists on optimal management, as much of the literature is limited to small case series. Surgical interventions to address patella baja range from tibial tubercle proximalization to patellar tendon lengthening or reconstruction. Allografts and autografts can be utilized to either augment tendon lengthening or reconstruct the patellar tendon. The purpose of this paper is to review the diagnosis and surgical management of patella baja and highlight the senior author’s surgical technique.

Evaluation of Patellar Height

A variety of different measurements exist to assess patellar height. In a normal knee, at 30 degrees of knee flexion, Blumensaats line should extend to the inferior pole of the patella. The Insall-Slavati (IS ratio), Blackburn-Peel (BP), and Caton-Deschamps Index (CDI) are some of the more commonly used methodologies for evaluating patella alta and baja [4–6], The CDI was first described in 1982 to better understand the relationship between low-lying patellas and decreased knee range of motion. The measurement compares the articular surface length of the patella to the anterior superior angle of the tibial plateau, with a value of <0.6 indicating patella baja (Fig. 1A) [7]. While there is significant variability among individuals, patellar height is consistent between the same individual’s knees. Thus, a 10–15% decrease in CDI from the contralateral side can also be considered patella baja [8].

Fig. 1.

Lateral knee radiographs and MRI demonstrating the measurement of CDI. A and PTI. B in a 59-year-old female with patella baja status post ACL reconstruction

While there is no consensus on the best measurement for patellar height on the lateral radiograph, the CDI is the preferred method of the senior author. The CDI is reproducible on both radiographs and magnetic resonance imaging (MRI) and is a reliable measurement in both children and adults [6, 9–12]. Moreover, it is variable with tibial tubercle location and thus changes in this measurement are reflected status post tibial tubercle osteotomy.

Patellar trochlear index (PTI) is another valuable method used to evaluate patellar height that is determined on sagittal cuts of MRI. This measurement offers the ability to identify functional alta or baja in that it better characterizes the interaction between the patella and the trochlea. The measurement is determined by comparing the ratio of the measurement of the amount the trochlea engages with the patella to the patellar articular surface (Fig. 1B). A ratio of greater than 50% indicates patella baja and less than 12.5% indicates patella alta [13]. Studies demonstrate good to excellent intra- and inter-observer reliability of this measurement [13, 14].

Finally, as patella baja is often seen status post total knee arthroplasty (TKA), modifications of both the IS ratio and CDI have been made to improve the assessment of patellar height in the setting of arthroplasty [15–17]. As the tibial reference point in the CDI is resected in total knee arthroplasty, new tibial reference points have bene proposed on the tibia both independent and dependent of the prosthesis [16, 17]. Recently, a “derived Caton-Dechamps Index” (dCDI) has been developed using the anterior proximal point of the inlay as a reference point with high intra- and interrater agreement [17]. This measurement can also detect changes in patellar height relative to the joint line, or pseudo patella baja.

Iatrogenic Causes of Patellar Baja

Acquired patella baja results as a complication of traumatic knee injuries and surgical intervention with a multifactorial etiology. A combination of quadriceps weakness, traumatic injury, alterations in native anatomy, and immobilization lead to tendon shortening and loss of patellar height. Iatrogenic causes can be differentiated into acute versus chronic; acute changes occur immediately following injury or surgery while chronic changes lead to progressive inferior displacement of the patella [18].

Chronic patella baja occurs as a late manifestation of infrapatellar contracture syndrome (IPCS) following knee surgeries such as anterior cruciate ligament (ACL) reconstruction, meniscectomies, ORIF for fracture, and total knee arthroplasty (TKA) [18–20]. Paulos et al. describes a prodromal stage during which periarticular inflammation, immobility, and quadriceps weakness lead to decreased patellar excursion, painful active range of motion, and an extensor lag. The second stage, or active stage, is characterized by a sharp decrease in patellar mobility and lack of both active and passive knee extension. Patella baja manifests in the third stage, often accompanied by patellofemoral arthrosis [18, 20]. The etiology is thought to be a combination of immobilization, inflammation, and quadriceps weakness.

Ahmad et al. further elaborates on the effect of patellar tendon adhesions on knee mechanics and their role in patella baja. By simulating patellar tendon adhesions on cadaver specimens, they found that these adhesions lead to medial and distal translation of the patella and a decrease in patellar tendon length. This led to both diminished extensor mechanism strength and alterations in the biomechanics of the patellofemoral joint suggesting increased joint reactive forces [21]. The etiology of these adhesions is postulated to be injury and subsequent scarring of the infrapatellar fat pad during surgery or trauma [21].

Patella baja has also been reported as an acute complication of fractures of the knee including those of the patella, distal femur, and tibial plateau [18, 22, 23]. While classically the mechanism was thought to be inflammation and adhesions as described in infrapatellar contracture syndrome, Mariani et al. reports on a series of fractures about the knee with acute rather than chronic onset of patella baja. In their series, the authors found the persistence or worsening of patella baja at follow-up. They hypothesize the mechanism of acute patella baja to be due to injury to the articularis genus, which is responsible for elevation of the capsule and synovium [18]. However, case reports of patients with delayed patella baja following retrograde nailing support the theory of infrapatellar fat pad scarring as a contributing etiology.

High rates of patella baja have been reported following ACL reconstruction, particularly those in which a patellar tendon autograft was utilized [8, 24, 25]. Studies demonstrate that shortening of the patellar tendon occurs following patellar tendon autograft harvest, suggesting that both an alteration in anatomy along with infrapatellar adhesions contribute to patella baja [26, 27]. Patellar tendon shortening following ACL reconstruction, however, has not been correlated with anterior knee symptoms [24, 25].

Multiple studies report postoperative patella baja following high tibial osteotomy (HTO) [28–31]. Otsuki et al. found a linear correlation with HTO medial opening wedge correction angle and the postoperative decrease in CDI [28]. Osteotomies above the tubercle are associated with a greater risk of patella baja than those preformed below the tubercle. As infrapatellar osteotomies spare the patellar tendon, they are thought to cause less injury and subsequent fibrosis to the tendon [31]. Patella baja can occur due to over distalization of the tibial tubercle or as a late complication of tibial tubercle osteotomies [32].

Finally, TKA is another significant cause of patella baja, which can be due to either patellar tendon shortening or alterations in the joint line relative to the patella (pseudo patella baja). At five years postoperatively, 34–38% of total knee arthroplasties demonstrate 10% or greater shortening of the patellar tendon [33, 34]. Patellar shortening increases between 1 and 5 years postoperatively, which suggests a chronic process of quadriceps weakness and infrapatellar adhesions similar to that described in infrapatellar contracture syndrome [2, 20, 34]. Alternatively, a large polyethylene insert, often used to correct instability, elevates the joint line with respect to the patella, effectively creating patella baja. This complication is relatively common, occurring in approximately one quarter of TKAs [35].

Treatment of Iatrogenic Patella Baja

The treatment of patella baja can be challenging, as often patients have undergone multiple prior surgical procedures. Non-operative management can consist of physical therapy, aggressive range of motion, systemic steroids, extracorporeal shock wave therapy, and dry needling with limited success [36]. Initial aggressive postoperative range of motion and muscle stimulation, particularly quadriceps muscle groups, is imperative for prevention of the development of patella baja postoperatively [12, 20]. Surgical options for patients with symptomatic patella baja include excision of the lower third of patella, lengthening of the patellar tendon, reconstruction of patellar tendon with allograft, tibial tubercle proximalization, and in salvage cases, patellectomy [36, 37].

Proximalization of the tibial tubercle has yielded positive results in the treatment of patella baja in a few small cases series [12, 37, 38•, 39•]. Drexler et al. describe a technique that utilizes a lateral parapatellar approach to access the knee. They perform a 2-cm wide × 1-cm deep × 5–10-cm long osteotomy to mobilize the patellar tendon. The tubercle is moved 1.5 cm proximal and secured with two large fragment screws or in cases of poor bone quality, a five-hole semi-tubular plate with screws. In their series of 15 patients who underwent proximalization, the authors report excellent correction of BP ratio and significant improvements in patient-reported outcome measures (PROMs). The mean preoperative BP ratio was 0.4 and improved to a mean of 1.0 postoperatively [37].

Schmidt et al. discuss a similar osteotomy performed in conjunction with an arthroscopic lysis of adhesions and medial and lateral retinacular lengthening. Notably, they perform a V-shaped osteotomy for mobilization proximally and fix their osteotomy in 120–130 degrees of flexion to approximate 1.5 cm proximalization. In their case series of 11 patients, the CDI and BP ratio normalized postoperatively. Significant improvements were seen in their range of motion, self-reported knee function, and other PROMs [39•].

Proximalization of the tibial tubercle is also described in the setting of patella baja secondary to chronic quadriceps rupture [40]. Correction of the patella baja at the same time as quadriceps repair is thought to offer a more optimal healing environment for the quadriceps tendon. Furthermore, this technique has been applied to patients with pseudo patella baja in the setting of TKA [38•, 41]. The authors found that better outcomes were achieved in patients who underwent proximalization greater than 1cm. Good outcomes were reported in the osteotomy group with significantly higher improvements in Knee Society Sore in this group compared to the controls [38•]. A concern with proximalization is that it limits exposure in the case of future TKA as the patellar tendon insertion is closer to the joint line or at the level of the tibial baseplate.

Patellar tendon lengthening is another surgical option for management of patella baja, with better results seen in patients with more severe preoperative patella baja [42, 43•]. Originally described by Dejour et al., Wierer et al. describe a modified Z-plasty in a case series of four patients with patella baja. After an initial arthroscopic lysis of adhesions, a longitudinal incision is utilized to expose the patella tendon. The patella tendon is incised longitudinally and each limb is split into a superficial and deep layer to create four total limbs. The medial superficial and lateral deep limb are detached from the patellar insertion and the lateral superficial and medial deep limb are detached from the distal insertion on the tibia. After mobilization of the patella, the tendon limbs are reapproximated with sutures. The four-limb technique provides approximately four times more surface area of tendon overlap than conventional Z-plasty. Postoperatively, patients in their case series demonstrated normalization of the CDI with improvements in range of motion and no extensor lag [43•].

Successful lengthening of the patella tendon has also been described using an Ilizarov technique in which an external fixator was applied to the patella and the tibia. In the case report, initial lengthening of 1 cm was obtained on postoperative day one followed by an increase of 1mm/day. After obtaining 38 mm of lengthening, the frame was kept in place for 3 months to maintain length. The authors report near normal patellar height from a CDI of 0.38 preoperatively [44].

Allografts can be utilized to both augment lengthening and to reconstruct the extensor mechanism. Jeong et al. report on a case in which Z-plasty lengthening was augmented with a tibialis anterior allograft. Following patellar tendon Z-plasty, the tibialis anterior allograft was prepared to pass through a 7-mm graft sizer and each end was whipstitched with Ethibond suture. Two parallel 7-mm tunnels were created in the patella and a transverse tunnel was created in the tibia. The graft was pulled through the tibial tunnel and each end of the graft was then passed through the patellar tunnels. After adjustment of the tension and patellar height, the sutures were tied over the patellar bony bridge [45].

Finally, case reports of extensor mechanism reconstruction of the patellar tendon with fresh-frozen Achilles allograft have been described in patients who failed prior extensor mechanism surgery [46]. In this technique, after complete release of the medial and lateral retinaculum, the patella is completely detached from the tibial tuberosity. The patella height is corrected to 1.5 cm proximal to the joint and Achilles allograft with a calcaneal bone block is draped over the patella and quadriceps tendon. It is fixed distally via the calcaneal bone block with cortical and cancellous screws and sutured to the patellar and quadriceps tendon proximally with non-absorbable sutures. In their case report, Yoo et al. report postoperative improvement in knee range of motion with no extension lag and normalization of IS ratio [46]. Other methods for reconstruction include use of patellar tendon allograft or contralateral knee BTB autograft (patellar bone-patellar tendon-tibial bone) [47].

Preferred Technique for Patellar Tendon Lengthening

The senior author’s preferred technique for the surgical management of patella baja involves a Z-lengthening with a semitendinosus autograft. Indications for treatment include symptomatic patella baja with a CDI <0.8, although most cases have a CDI of <0.6. Many of these patients have limited knee range of motion, anterior knee pain, and significant weakness with active knee extension.

The patient is positioned supine with a sterile tourniquet. A diagnostic arthroscopy is first performed, and any intraarticular pathology is addressed at this time. An arthroscopic debridement of any hypertrophic synovium and adhesions is performed. Once completed, the arthroscope is removed, and the knee is drained of all fluid. The tourniquet is then inflated, and an anterior longitudinal incision is made down to the patellar tendon and the hamstring tendons. Frequently, these patients have had prior surgery and if possible, prior incisions should be utilized. The semitendinosus is then identified and harvested using a tendon harvester, leaving its distal insertion on the tibia attached. The gracilis is left intact.

Attention is then turned to the patellar tendon. The tendon is released medially and laterally from the medial and lateral retinaculum. Beginning at the tibial tubercle, a transverse incision is made from lateral to medial through 50% of the tendon and then carried proximally towards the patella. Proximally, the posterior aspect of the tendon is detached from the patella creating both a distal and proximal limb (Fig. 2A and B). The inferior bony surface of the patella is debrided with a rongeur and two all suture anchors are placed in the medial and lateral aspects of the inferior patella.

Fig. 2.

An AP and Lateral schematic demonstrating a transverse incision from superficial to deep through the patellar tendon that extends proximally, detaching the posterior 50% of the tendon from the patella (A). The superficial fibers are kept attached proximally and the deep fibers are kept attached distally. The tendon is lengthened and then reapproximated in an end to end fashion (B and E). The hamstring autograft (shown in orange) is sutured to the medial and lateral tendon edges and secured to the patella with suture anchors (denoted by black circles) (C and F)

The patellar tendon is then released from the scarred infrapatellar fat pad to provide adequate mobilization of the tendon (Fig. 3A). With the knee in 90 degrees of flexion, the tendon edges are then reapproximated and the desired length of the tendon is established (Fig. 2B). In most cases, the edges of the patellar tendon are reapproximated end to end with the goal of a CDI of approximately 1.0–1.1. Once the length is set, the semitendinosus graft is sutured along the medial aspect of the patellar tendon from distal to proximal using a Krackow #2 non-absorbable high strength suture and secured to the medial suture anchor. The graft is passed anterior to the patellar tendon and secured to the lateral suture anchor. Finally, the graft is secured to the lateral border of the patellar tendon, again, with a Krackow #2 non-absorbable suture (Fig. 2C). The final construct of the hamstring autograft augment is demonstrated in Fig. 3B and C.

Fig. 3.

Intraoperative photographs of patellar tendon lengthening procedure following extensive release of extensor mechanism (A) and subsequent reapproximation, lengthening, and augmentation with hamstring autograft of patellar tendon (B and C)

Following tendon and autograft fixation, the knee is taken through a range of motion and the degree of flexion at which point the patellar tendon is noted to be on tension is documented for postoperative range of motion restrictions. The wound is then irrigated and closed in a layered fashion (Fig. 4).

Fig. 4.

Pre- (A) and post-operative (B) lateral knee radiographs of the patient in Fig. 3 demonstrating a significant improvement in patella baja. The CDI increased from 0.56 pre-operatively to 1.1 post-operatively

Postoperative Rehabilitation Protocol

Postoperatively, flexion is limited to the maximum flexion achieved in the operating room without undue tension on the repair (typically 80–90 degrees). A hinged knee brace with quadriceps electrical stimulation is utilized postoperatively to help encourage quadriceps function. Patients are allowed to weight bear as tolerated locked in extension for the first 6 weeks. CPM is typically initiated on post op day 1 at 0–60 degrees. Range of motion is progressed as tolerated up to 90 degrees by week 6. Formal physical therapy includes patellar mobilization medially and laterally as well as blood flow restriction therapy with quad sets, which begins 2 weeks post-operatively.

Conclusions

Iatrogenic patella baja is a complication of knee trauma and surgical procedures and has a multifactorial etiology. While acute patella baja is more likely attributable to alterations in native anatomy, chronic patella baja involves a complex interaction between quadriceps dysfunction, immobilization, and inflammation leading to infrapatellar scarring and adhesions. Tibial tubercle proximalization osteotomies, patellar tendon lengthening with or without graft augmentation, and patellar tendon reconstruction comprise viable treatment options for patella baja. A comprehensive evaluation of the outcomes of surgical treatment, however, is limited by the lack of larger case series. Patellar tendon lengthening with hamstring autograft is the preferred surgical technique of the senior author.

Declarations

Conflict of Interest

Kathryn Barth and Sabrina Strickland declare that they have no relevant conflicts of interest.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by the authors.