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Nikita Ignatiev
Nikita Ignatiev

Talus



Bear Gulch Cave at Pinnacles National Park was formed by rockfall events infilling an existing canyon most likely during the last ice age. Stairs original constructed in the 1930s by the CCC, provides ease of access and leads visitors into one of the best examples of a talus cave in the National Park Service.




talus



Pseudokarst is a landform that occurs in many rocks, including igneous and metamorphic, and includes features such as hummocky topography, sinkhole-like closed depressions, and even caves. Pseudokarst caves form through mechanical processes rather than the chemical dissolution of bedrock that occurs in rocks like limestone. One type of pseudokarst cave forms within rubble shed from steep sided or sheer exposures of bedrock. This rocky rubble is known as talus when the size of the boulders is above a certain threshold; rubble mostly of pebble sized rocks is generally known as scree. However, geologists will often use the term interchangeably.


Talus or boulder caves can be found in areas with predominantly competent rock types, such as granites or gneiss, or any fractured rock that forms large, angular boulders. Talus caves are more commonly located in the mountainous states of the American West and the Appalachian region, but can be found throughout the US wherever appropriately sized rocky debris can accumulate as a pile. The passageways found in talus caves are highly variable and can twist and turn, alternating between tight crawls and squeezes with many entrances. Rooms can be large and some caves have been surveyed for more than a kilometer.


The known talus caves of the National Parks are few in number but are spectacular examples of this type of pseduokarst. In Yosemite National Park, voids between large, angular blocks of granodiorite have created caves at the bases of the iconic cliffs. Short but complex passages can wind for upwards of a 100 meters or 300 feet though the vast rubble piles. In steep, mountainous areas, weaker rock such as rhyolite can pile up in narrow canyons and form talus caves. Pinnacles National Park in California has several talus caves that formed this way during the last ice age. Tectonic fractures and narrow canyons were filled with rhyolitic debris. Smaller rocks and gravels were removed by flowing water leaving large boulders wedged in place and forming a series of interconnected voids or rooms. The talus caves at Pinnacles are known for some of the largest room-sizes in the US. These can be explored by trails and stairs constructed by members of the Civilian Conservation Corps during the 1930s.


Talus caves can form rapidly, sometimes from a single large rockfall event, though these young caves are very unstable. The red-hued cliffs of sandstone in the iconic desert southwest parks can form talus of sufficient size and shape to create talus caves. These caves tend to be short, tight, and small with lots of loose rocks. A large rockfall in Glen Canyon National Recreation Area in the last decade formed two such caves. One cave was obviously a danger, threatened by continued movement as boulders settle and slide from gravity and weathering. The second cave was observed to be a single space, 0.5 m (1-1/2 ft) high under a house-sized sandstone slab.


This rare community is known only from the southwestern corner of Wisconsin's Driftless Area. Algific talus slopes are small and isolated and tend to occur on steep north- or east-facing slopes with a substrate of fractured limestone (dolomite) bedrock that retains ice and emits cold air throughout the growing season. The community is dependent on water entering gaps in the dolomite, freezing in winter, and then slowly melting during the summer months and producing a steady outflow of cold air. Cold microhabitats support and enable the persistence of disjunct northern plant species, and periglacial relicts such as northern monkshood (Aconitum noveboracense) and globally rare terrestrial snails. The woody overstory is often sparse, composed of scattered, small black ash (Fraxinus nigra) and paper birch (Betula papyrifera). Mountain maple (Acer spicatum), a northern shrub, may be frequent, and extensive beds of bulblet bladder fern (Cystopteris bulbifera) and mosses are characteristic herbs.


First, the ankle is a highly congruent joint, which is important to know when using different methods for cartilage repair, such as autologous osteochondral transplantation. Additionally, the nature of the joint will affect the development of pain in osteochondral defects of the talus[11]. Of note, the average thickness of the talar articular cartilage is approximately 0.89 mm whereas knee cartilage thickness reaches 6 mm[12,13]. Moreover, the tensile stiffness of healthy talar cartilage has only minimal topographical variability and the dynamical stiffness is higher than in the knee[14,15]. A further difference is the lower contact area and the lack of absorbability that makes the cartilage able to tolerate higher maximum loads[16]. Additionally, its metabolic activity appears to be greater than that of the knee, with a higher turnover as well as a higher level of proteoglycan synthesis[16].


Donor-site knee morbidity could pose problems for patients, but it is not discussed in any of the published series[17]. Therefore, some authors suggest harvesting the osteochondral plugs from the talus itself to avoid donor-site knee pain, stiffness, or even arthritic changes[42]. Two series specifically addressed donor-site morbidity[43,44]. In a retrospective study of 11 patients, Reddy et al[44] showed that the number of grafts obtained had no effect on clinical outcome. Paul et al[43] found that a high body-mass index influenced the outcome score negatively.


Sagittal T1 and T2-weighted magnetic resonance imaging scan. A: Sagittal T1-weighted magnetic resonance imaging scan demonstrating deep osteochondral defect of the posterior aspect of the talus; B: Sagittal T2-weighted magnetic resonance imaging scan showing the several cystic lesions of the talus in addition to an osteochondral defect.


An immunologic reaction that adversely affects the chondrocytes, the limited availability of grafts, and the acceptance of costs may be further disadvantages[47]. Several authors have investigated the treatment of large osteochondral defects of the talus using osteochondral allograft transplantation in case series[7,46,49-52]. The overall clinical results were promising, especially considering the size of the defects. However, in certain of these studies, only a few patients were reported to be symptom-free[51]: some patients needed further surgical treatment, or the procedure failed[46,49,51].


Autologous chondrocyte transplantation/ implantation: Brittberg et al[54] implemented the technique of autologous chondrocyte transplantation in 1987. The first results were published in 1994 after treating chondral defects of the knee with this technique. Since then, it has become a promising tool for the repair of cartilage defects. Several long-term trials have provided strong evidence of the efficacy of this procedure, primarily studying its application in the knee[55-57]. Young patients suffering from a single focal cartilage defect with only a short duration of symptoms should expect good results[58]. However, to our best knowledge, equivalent data do not exist regarding the treatment of the talus. Additionally, a clearly recommendation regarding the defect size in which this procedure works best cannot be given: reported defect sized vary between 2 cm2 and 12 cm2[59].


Methods: Fifty patients with chronic osteochondral lesions of the talus underwent arthroscopic treatment. Average age was 32 years (range, 12-72 years). Average follow-up was 71 months (range, 24-152 months). Treatment consisted of either drilling of the osteochondral lesions of the talus in situ (n = 4), excision of the osteochondral lesions of the talus and abrasion arthroplasty (n = 6), or excision of the osteochondral lesions of the talus and drilling (n = 40). Preoperative and intraoperative staging of the osteochondral lesions of the talus was performed. Follow-up evaluation included 3 clinical rating systems: Alexander, modified Weber, and American Orthopaedic Foot and Ankle Society Ankle/Hindfoot scores.


Conclusion: Arthroscopic treatment of chronic symptomatic osteochondral lesions of the talus results in good clinical outcomes in the majority of patients. However, pain and functional limitation may persist in some patients, especially those noted to have unstable osteochondral defects at the time of arthroscopy.


Osteochondral lesions of the talus can present as a late complication of ankle injuries. As the talus is largely covered by articular cartilage, it has a limited ability for repair. Early and accurate diagnosis is important as talar integrity is required for optimal function of the ankle. The common presentation is chronic ankle pain with a history of ankle trauma. Conservative treatment involving a period of casting and non-weight-bearing is recommended for acute, non-displaced osteochondral lesions. Surgical management is recommended for unstable lesions or failed conservative management.


The Patient Specific Talus Spacer is an additively manufactured implant intended to match the patient's talus, a bone in the ankle joint that connects the leg and foot. The implant allows a patient with avascular necrosis (AVN) of the ankle joint to regain motion and reduce pain without an amputation or fusion until the time a fusion is necessary. The Patient Specific Talus Spacer is made from cobalt chromium metal alloy.


The Patient Specific Talus Spacer provides probable benefits related to improvement in pain with AVN talus patients who have limited treatment options and are high risk for fusion or amputation. The clinical data found the rate of reoperation was low, with 9.4% of cases (total of 32 cases, 31 patients) resulting in reoperation or correction. 350c69d7ab


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